Well, my black band is going shiny on the high wear parts of the tail, but remains matte elsewhere. Having washed it thoroughly this effect remains. Are you certain that Apple aren't using any coatings?
I am in the rubber injection business and knows the company that makes the bands for Apple.
Maybe it was a batch of bad mixed polymer but I am sure they do not coat the rubber.
Usually blooming occurs when the rubber compression time was shortened.
Another cause is excessive wax. Wax is what's giving the band a nice smooth feel and also to help the band come off the mould easier (because it's less sticky), but it will make it's way to the surface if too much wax was added (visible in black band, will be difficult to see in other colors).
If you see blooming on your black bands, touch it, if it feels smooth and dry, then it's really not an issue (not counting aesthetics), but if it's sticky, then you got bad rubber, definitely exchange it.