black is the "perfect" radiator and adsorber of heat. White is the reflector of heat.

For a water-cooled motor, the radiated heat from the block is small compared to the heat dissapated by the air/water radiator.

So, to answer your question, coating the block flat black theoretically helps. But paint can be an insulator too!

Like C9x said, air cooled bike engines need all the help they can get so flat black is the solution for them.

mike in tucson