If I were making my living designing and selling helmets I'd aim for the mass of helmet users, which doesn't tend to ride at night. I'd also probably watch the latest trends in riding apparel and try to make my helmets look good with what the norm in cycle wear is. If black helmets were the big sellers I'd be sure to have a bunch in my line-up.

I taught the MSF course for motorcycle riders a couple of years and told every class that a bright colored helmet is a great improvement in visibility and I know that to be true. That said, I have a dark yellow BMW R1100S with matching dark yellow and black jacket with minimum reflective piping and two black helmets (one flip up). I've been riding with this gear since I bought the bike new in 2000 and I've never had a close call that I didn't see coming and adjust for. Awareness and the ability to react pays off in spades. I use lights for visibility and keep my awareness up with the help of a mirrors. Bike commuting is different in that there's a great deviation in speed between the rider and traffic. For that reason I'll add a couple of extra tail lights but hold short of looking like rolling Christmas tree.

In situations such that a big city brings, all the visibility you can get as a cyclist may pay off. That's not my commute.