most are black because that's what absorbs photons. asphalt happens to be black, but it converts 100% of captured energy into heat, whereas PV systems decrease that value to 95-50% (source: http://en.wikipedia.org/wiki/Solar_cell_efficiency#mediaview...)
They're black in visible light. But as far as I know they cannot harvest IR photons. So it would probably be useful if they reflected IR.
Although I don't know if it's better for them to be reflective or black in the IR window where where atmosphere is transparent. Black makes for good emitters too and that way they could shed heat into space.
If we could arbitrarily designed reflectivity vs. transparency it would probably be transparent in visible, reflective in IR, black in the IR window
