Maybe what we need is a fan to cool down the fan.

Yeah the fact that it's spinning as much as it is would probably dissipate any excess heat that black paint would add. Lots of airflow. My guess, at least.

Could be an issue when it stops spinning though.

Does that happen often? When I pass by the wind farm every time I go to visit extended family, I don't think I've ever seen them stopped. Granted that's not often. I really don't know.

I imagine you'd place them in an area where there's almost always wind, otherwise you've planned poorly.

Yeah it's a possibility. The black blade might expand more in the heat, so might become (for example) 10 mm longer than the white blades, which could cause a vibration problem due to being out of balance.

Currently, all the blades are the same color, so should expand and contract equally in the sunlight.

Probably more than those painted white, and another study could help answer that.

Maybe make it striped instead of solid black