heat is the likely cause?
I'm not sure what the best fix might be..
but it would seem to me that heat is the likely cause.. if it's exposed to the sun and painted black, it probably heats up and expands more than the other white areas..
I would think that you might want to fix that issue while you fix the cosmetic damage.. I don't think you'll want to paint it white (although probably the best fix), but maybe there's a special gelcoat sealer for high heat / expandability..
just a thought..