According to a chemist on /r/askscience, the plastics probably contain carbon black [0] as a UV protectant. So it might not be deliberate. (/u/Platypuskeep

According to a chemist on /r/askscience, the plastics probably contain carbon black [0] as a UV protectant. So it might not be deliberate. (/u/Platypuskeeper) Although I have not seen a specification of what the balls are made of, what I know from polymer chemistry is that the black is quite likely from carbon black (essentially soot), which is used as an additive/filler in polymers (e.g. tire rubber) for multiple reasons but not least UV protection. Carbon black is one of the most (if not the most) effective UV-protective additives, precisely because it absorbs the UV and prevents it from penetrating deeply into the plastic. Plastic without some UV protective additive would degrade pretty quickly in the California sun, not least HDPE which has pretty bad UV resistance. https://www.reddit.com/r/askscience/comments/3gpu6x/why_are_the_shade_balls_in_los_angeles_black_and/cu11268?context=3 https://www.reddit.com/r/askscience/comments/3gpu6x/why_are_... [0] https://en.wikipedia.org/wiki/Carbon_black https://en.wikipedia.org/wiki/Carbon_black