Wouldn't floating white spheres make more sense ? If the goal is to avoid overly warm mater.
Veritasium video about Los Angeles' reservoir black spheres project. They used black because
- The goal was light-blocking more than anything else; black worked best
- The black pigment held up to the environment (especially UV degradation) better than anything else they tried
I expect the second point was the most important thing. Black plastic is very stable.