Although, you're totally right to criticise the company, despite their honesty and possibly clumsiness (as they could have feigned a better excuse not to hire you); do be aware that protesting as a foreigner will do little to change racist attitudes in China or any Asian country.

I'd suggest just moving on from them, as there are schools where being black could arguably make you more marketable; that's not a racism thing, but in a country with far too many ESL teachers, anything that makes you stand out from the crowd is probably a bonus. Even if racial stereotypes mean that half the kids will expect you to be great at basketball and like rap music or something.

But most importantly of all, if somebody doesn't want you, you're probably better off without them. If they'd hired you, you'd probably become their whipping boy whenever their profits had fallen. Go somewhere where you are valued instead.