"Pure black carbon gives the balls their color after being added to the plastic to absorb UV rays. That prevents sunlight from penetrating the plastic and getting to either the water inside the balls or beneath them. A lighter-colored ball may allow sunlight to pass through into the water, rendering it ineffective, said Sydney Chase, who owns shade ball manufacturer XavierC, one of the suppliers involved in the project."

http://www.nbcnews.com/storyline/california-drought/great-balls-water-millions-shade-spheres-afloat-l-reservoirs-n409306

From comments on a lot of sites I see a lot of reasoning as to why black is better, but there's something from an actual article. The carbon black seems to block UV better. That seems to be the most common reason. There are various other reasons mentioned (including they might last longer), but I don't know what is true.

Oddly enough, this came from a guy named White:

"The Los Angeles utility is the first to use shade balls on a large scale. The idea came from now-retired LADWP biologist Brian White, who was inspired by the "bird balls" used to deter birds in ponds along runways."

http://news.nationalgeographic.com/2015/08/150812-shade-balls-los-angeles-California-drought-water-environment/

As for the chemicals, that's something I was thinking about too. (That seems to be a very common concern from everyone about this project.) I don't know how much better this plastic is compared to others. Some plastic things are more okay than others, but it is really hard to tell with most things. (It's not going to say it on the bottle.) I like to use the same water bottles over and over again and have them in my fridge filled with tap water so the water is always nice and cold. I prefer using the same bottle partly because I think that whatever might leach out will be less and less over time than using new bottles more often. More from that bottle over time, but less perhaps than if I constantly used new bottles. Plus I don't want to use new bottles all the time, although I do also recycle. (I don't like glass, I'm too clumsy, and I don't like a cup filled with water, regardless of what it is made of, because it could be too easily spilled in the fridge while getting cold. I like it capped.) I don't change the bottles too often. I'll wash them out and replace them if they break. In reality though, the levels of what is released is probably not going to ever have any impact on a particular person. (For example, if there was a one in a million chance of getting ill, some alternative might have just as much, if not more, risk.)

On a side note, the Internet has failed. How have we gone seven years and not seen this yet? Social media is all over this now, but it takes the last 20,000 out of 96 million before it goes viral? They could have sold tickets to this all this time! They could still have a viewing area behind glass and charge admission. This would totally be a money maker!