As Bismark says, there really isn't any DIR preference for black. In fact, you'll see a lot of WKPP folks diving DUI drysuits with the red inlays! I used to have a blue and white dry suit, and I dive a bright orange wing, and nobody has ever said anything to me about my motley outfit.
You have a good question, though, about why DIR DOESN'T recommend or standardize on high visibility colors, because one would think that such a safety-oriented agency would. We do, however, carry SMBs routinely for open ocean diving, and they're orange and very visible. I suspect the reason yellow isn't mandated is that, with the training we get, separation and searches are rare. That's not to say they're unknown, but it simply isn't enough of a problem to motivate a standards change.
Lots of us don't pass . . . the first time. The value of the class is what it teaches you, and how your diving subsequently changes. Passing is necessary only for going on to GUE tech or cave classes.