If you were on the gold colored cavern line at Taj, there is visible natural light along the entire length of it, but in the middle (the line route is shaped like a very long and skinny "U") it's very faint, and you wouldn't see it unless there were no other dive lights around and you gave your eyes time to adjust. It would appear as a very faint deviation from the absolute pitch blackness of zero light. I dive on that line frequently because it leads to a few very beautiful cave passages. It's a pretty long cavern line. I would be interested to know if someone has actually measured the distance from the farthest point in on the line to the surface of an exit. If I remember right, that must be no more than 130 ft. It seems pretty far to me.
Still, dozens of people are guided on that line every single day of the year, year in and year out, and you don't hear about divers getting lost or dying. That means that generally speaking, the cavern tour guides are doing a decent job and the dives are relatively safe for OW divers. It's pretty well regulated, and there are cave instructors in there every day. If they see something amiss on the cavern tours, usually they are not shy at all about speaking up about it.
I'm glad you had a good time, and it sounds like you'll do more. You might even consider taking a cavern course with one of the cave shops down there. My favorite is the one that is right at the entrance to Taj, (at least for a few more months until they have to move) It's called Under the Jungle.