I brought back some of these from China a few years ago, and pondered the same question in the company of a Japanese friend.

She explained that she uses black q-tips because they allow her to see her earwax more clearly than the white ones.

I brought out the white cotton swabs and we did a series of tests and sure enough, her earwax was whitish, powdery, and showed better on the black swabs. My non-Asian, more waxy earwax did not show very well on the black swabs. It showed on the white swabs, and hers, did not.

So yeah, there's your explanation. Sorry if it's kinda gross.

I like how "Vari-colored" used to be a big selling point.