Is it possible to make broad spectrum light absorbing paint?
Carbon nanotube black is highly absorbent in and outside the visible spectrum, see http://www.extremetech.com/extreme/186229-its-like-staring-into-a-black-hole-worlds-darkest-material-will-be-used-to-make-very-stealthy-aircraft-better-telescopes.

Of course, that does not make it ideal for energy conversion, as it would also need at least the following qualities to be practical.


A good conductor of heat to the heat-exchanger surface below
An enduring finish, resistant to abrasion, heat and environmental degradation
Economical, both in material cost and application
Safety in use and in manufacture