Rob,
I've been diving a very long time (I started in 1959), and may have some perspectives which would help you. First, I'm putting a photo up so others can see it. I took it at the U.S. Navy School for Underwater Swimmers in Key West, Florida in 1967.
The photo shows a diver in a double tank with a double hose regulator standing on a dock in Key West, ready to enter the water. He has a canvas bag in his hand. What people viewing the photo cannot see is that the bag contains parts for an assembly task. This is a test of dexterity, the ability to manipulate and assemble things underwater, and solve the problem of getting the parts together. What is also not shown in the photo is that this was in a very dirty harbor, where the visibility is zero underwater. The diver was not to surface until the part was assembled. For some of us, it took only about ten minutes. For others, it was nearly impossible as they were totally dependent upon vision. But with this test, we had to do everything by feel. The U.S. Navy thought this was so important that the future graduates of the Underwater Swimmers School would not graduate until they had mastered this skill.
Diving blind is about like diving in zero visibility, but it presents some unique problems. These include not being able to see on the surface to determine where things are. It is also important to maintain buddy contact; in the U.S. Navy School for Underwater Swimmers, we used a "buddy line." This was a line about four feet long which was attached to each diver. If we surfaced without this contact, we got to use a different "buddy line" for the next 24 hours; this was a line that was 6 inches in diameter and about 10 feet long. We had to sleep with it too. This was the importance that was placed on maintaining buddy contact in all conditions. I used that on some challenging dives, including one off the Oregon coast where my buddy and I were rolled by a 20 foot wave, and had to stay out in the water until picked up by the U.S. Coast Guard. I'm showing a photo of that dive, taken by our girl friends, just after the wave rolled us.
This photo shows foam-streaked water, with the two of us together on the choppy surface coping with the situation. I had my helmet on (which was what got us spotted by the Coast Guard), but my buddy had lost his (we both dive helmets; mine a white-water kayak helmet and his a modified bicycle helmet). We stayed together because of that 1/4 inch nylon buddy line secured to our harnesses.
You are justifiably concerned about instrumentation. Please know that the original Cousteau patent on the Aqualung shows a three-cylinder configuration. Cousteau never did dive with a gauge for pressure. He relied upon his original concept of a spare cylinder as a reserve, and in the patent it stated that because of this configuration, as the two cylinders are drained of air, the diver will find it hard to breathe. He can then open the valve on the third cylinder as a reserve and swim to the surface. Because of this, "the diver can thus dispense with any instruments for measuring the pressure in the two cylinders A and B." (United States Patent #2,485,039, page 1 line 19-20). As the Aqualung was marketed in the United States, valves were made to replace the third cylinder. These were known as J-reserve valves. The restriction in the air supply was accomplished at 300 psig for a single cylinder scuba, and 500 psig for a double tank scuba.
Some manufacturers incorporated a reserve design into their single hose regulators. Healthways was a pioneer in this, and used three designs. One was known as a "restricted orifice," and was used even on the early Healthways double hose regulators. It had a smaller opening than other regulators, so that at about 300 to 500 psig, the breathing became harder. Going up lessened the pressure, and therefore the restriction. This regulator reserve mechanism forced the diver to the surface. But it was not good if the diver had to go down before surfacing (in wreck or cave diving, for instance). The second was incorporating the J-reserve mechanism into the regulator, and I've already discussed that aspect. The third is what is now known as a "sonic reserve." This is where the regulator, at about 500 psig, starts making a noise and/or vibration, which lets the diver know that he or she is getting low on air. It also alerts other divers in the area that the diver is low on air. What made it unpopular is that some say it attracted sharks in some tropical areas; this is unconfirmed on my part. Both Scubapro and Healtways made these sonic reserves, and I have collected examples of both. They are very effective warning devices for being low on air.
That brings us to the depth issue--knowing your depth. This could be handled by using a float with a weighted line, marked at intervals. The markings could be in braille, for instance.
Finally, there is the time issue. That could be handled in a couple of different ways. One, which was used extensively in the early days of diving, was simply to have an air supply which did not have enough air to get a diver into decompression trouble. The steel 72 cubic foot tank was thought to protect the diver from overstaying, although with more modern decompression algorithms it is certainly possible to overstay on that air supply. A second is simply to have a buddy with his instruments available immediately; here the buddy line would help.
Finally, I have the thought that one of my dreams since about 1972 was to get underwater communications going. That is now possible, with the full-face mask and communications systems, there would be no problem sharing information on depth, bottom time, decompression, etc. with a buddy in voice communication with you.
I have thoughts on BCs too. These devices, which I helped develop, were brought into being to compensate for the loss of buoyancy of a wet suit. But adding air to a device underwater could cause additional problems too. I know that all the agencies require BCs be used, but my thought is that for about 30 years of diving they were not used, and for a person who is blind they may create as many problems as they solve. For instance, sensing an increase in buoyancy underwater depends either on sight, or feel. But if the diver is wearing a wet suit, then feel in a problem (the suit covers much of the skin), and there is no sight. So a diver can get positively buoyant and begin an ascent without knowing it if he or she is blind, and is wearing a wet suit. Here, if a BC is used, it would be best to have a positive connection to the buddy diver (buddy line). I am what is now known as a "vintage diver," and many times dive without a BC. But I do like BCs enough to have created my own too (the Para-Sea BC, which is not commercially available). What I am saying though is for a blind diver, learning to dive may be easier initially without the BC. Melvin Pasley may have more information on that, and I'm sure it is against the diving agency's protocols, but it may also be something to consider.
These are some ideas I hope will spur you to take the plunge, to get into diving further, and enjoy this wonderful medium. I have had a number of "blind" dives in the past, including one into a dump to try to find a shotgun. The hunter was in a boat, and got excited when the ducks flew overhead. He stood up, shot at the ducks, and the force of the shot knocked him over the small boats gunwale. We dove for about half an hour, and finally gave up finding the shotgun. I did not completely remove my wet suit at the site, and came to my apartment (I was a college student at the time), got into the shower, and got out of it there. I then showered for a long time, washing my body and my wet suit. My buddy did not take those precautions, and came down with some boils. That was my first, and I think my only, commercial dive. But we made a bit of money even if we did not get that expensive gun for the owner.
On another dive, a friend owned a lumber mill, and one of the guys on the log pond had fallen into the water off a small craft used to move logs. He came up sputtering, and trying to get his breath (he had a life jacket on) spit out his false teeth into the pond. I dove to try to find them, but there were too many derbies on the bottom to find them.
I have also made night parascuba jumps out of US Air Force planes, and had to cope with a parachute in the water in the dark. So diving blind is not such a difficult thing.
I think you'll enjoy diving a lot.
SeaRat