I don't like black market, but i can count on one hand (out of 10000 games) the games i have lost because
1) my opponent got a game-warping card early out of the black market
even though
2) my opponent had done an equal or worse job than me in building a deck that could play black market a bunch
The mild amount of unfairness introduced by the mechanic is just dwarfed by all the other unfairness in Dominion; this one has never bothered me. Contrast, say, games where my opponent just spammed tournament and then got a followers before i had a chance; this happens quite often.