Yes, black does absorb heat - it also radiates heat more effectively than silver. The crankcases may well be black but they aren't powdercoated.
The material is what I am questioning, not the colour... A layer of powdercoat will retain more heat in the engine compared to a bare finish or paint - of any colour. It's the material that is doing the insulating.
But hey, whatever - powdercoat it twice in pink for all the difference it makes to me.