“The shade balls used in the Los Angeles project are made of high-density polyethylene (HDPE) with carbon black additive to protect the plastic from ultraviolet radiation. Adding carbon black also prevents the formation of bromate, a suspected human carcinogen.

[...]

The black coating on the balls, called carbon black, is a petroleum derivative which is classified as a carcinogen if inhaled as dust. ”

So you have balls made with a known carcinogen (if inhaled as dust) to prevent formation of another carcinogen formed in water. I’m guessing they researched the risks already and it’s a net positive.