Strange how modern helicopter rotor blades are commonly made of composite fibre and they're painted black....
Composite structures used in aircraft are usually cured in an oven as part of the manufacturing process. Heating in-service to temperatures lower than that used for curing do not cause a problem, heating to higher temperatures may lead to temporary weakening of the structure and deformation.
Helicopter blades are likely to have been cured to at least 125 degrees C, wheras glider structures to perhaps only 54 degrees C. Because of the somewhat lower temperatures used for curing gliders they need to be finished in light colours. (No, I don't know why they don't use higher temperatures for gliders!)