They sound surprised.

They are completely missing the fact that they were most likely made somewhere that water was plentiful. The point wasn't to save a net amount of water nationally, it was to conserve water someplace it is scarce.

Mistakes are part of progress.

It seems to me that the balls would have saved a lot more water from evaporation if they had been bright white or even silvery instead of black. The blackness of the balls no doubt absorbed a lot of solar energy, heating up the underlying water and making it evaporate even more.
I'd like to know if any experiments were done comparing different colored balls to see which would be most effective in retaining water in reservoirs.

I assume the balls are made somewhere else, and have a lifespan longer than one year...

Ben, respectfully, if the point hadn't been to generate net water savings, water could have simply been transported to CA, avoiding the plastic and manufacturing expense altogether. This seems to be a clear case of someone not doing an even basic back-of-the-envelope materials balance calculation.

shouldn't the balls be white?

I'd say they were black to be UV resistant and also still be "food safe". Some plastics might be UV resistant but then leech chemicals into the water etc... A food safe plastic like PET in black would be the safest and cheapest option.

@PaleDale I have the contrary, that many black plastics are not safe. Some of them are black because they contain recycled plastics of various color, hidden in a black color. And with recycled plastics, it is hard to be sure no one contains any dangerous product.

First, like other said, they should have been white, to prevent evaporation due to added heat. Second, that water that was used to make the plastic doesn't just disappear into the ether... It gets either evaporated and rained down again, or dumped back into the water supply it came from.