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_...

[0] https://en.wikipedia.org/wiki/Carbon_black

Black absorbs more light (and, yes, heat), which means less light penetrates through it to heat the water directly. If you block one bright light with a sheet of thin white plastic, and another with a sheet of thin black plastic, you'll see a lot more light shining through the former. And do remember that the primary purpose is to block UV, not heat, so you want maximum light absorption even if it means greater heat transfer.

Chrome-mirrored balls would certainly be even better, but the whole reason they're using plastic balls is because they're dirt cheap. If you have enough money to fill your reservoir with disco balls, you'd be better off just building a gigantic awning.

The primary purpose of these is not to slow down evaporation. In fact they have a negligible effect on the rate of evaporation.

Their primary purpose is to absorb the light spectra that drives chemical reactions in the treated water to prevent carcinogenic bromate from being formed.

Most of the reporting on this, including this article, is pretty bad, and causing a lot of laymen to ask questions like yours, because it doesn't make sense as described in the reporting.

Thanks for making a bit of sense of this!

black is best suitable color to deflect UV rays.

Now you just said the opposite of what intuition suggests without any evidence, or even argument. Please elaborate and don't just heat up the discussion!

Black is the best colour to absorb UV. You want the UV to not reach the water, so you want whatever is on top of the water to not allow UV past.

UV + Chlorine = Bromide

Does that look like an explanation to you? I don't see anything about black. I don't see anything about white. I don't see anything about reflecting materials. I see something from chemistry, maybe?

Customer: "Why should I buy the product?" Developer: "Here we had a problem with the inheritance tree. We worked around it with a factory method."