This is quite simple. What makes Tournament a card that is quite often a powerful card? Mostly you open Tournament, and if you do it's almost always because you want to get a Province as soon as possible and connect it with a Tournament. Why? Because the unique prizes (often one or two of them, and often Followers if it's the only Curse giver and/or discard attack) are that powerful and important. At least in Tournament games how you go about getting that Province, and how you go about connecting, is based on skill. Of course, as always in Dominion, it's possible to just get lucky and hit $8 early and get lucky and connect without doing much sifting or trashing or engine building or whatever (or more likely, doing it less skillfully than your opponent). But that's more on the level of the normal possibility of luck undermining skill that exists in Dominion.
In Black Market games on the other hand, if there's one or several important cards in the same vein as Prizes, it's much more likely that one player can get an important card just by playing BM. It just as likely to happen in turn 3 or 4 as later (except that you might not get $5 and it might cost $5, or it might costs $6). As an example, if there is no trashing and no cursing and you get a curser, that's extremely impactful, often more so than Followers, because Followers slows down the player playing it too, with Estates. As another example, if it's the only trasher.
So I don't really see the big relevance of the possibility of building a deck that can play more BMs. Yes, that's often better than not doing it. But that's beside the point. The point is the likelihood of just getting that important card without doing any building, or doing it in a significantly less skillful and effective way than your opponent.
SheCantSayNo. You said, "any match-up between identical strategies is always going to be a coin-flip". This was the exact thing I told you not to say. I'll explain why again. Some cards introduce more luck in the game than others. We call these cards swingy. Swindler is one example. Nevertheless, whether it's a high skill, perfect predictability game or a crazy unpredictable Swindler bonanza, the fact is that if both players play exactly equally well, who wins will be a coin flip. To distill it further down, if two players each throw a die and the higher number wins, or if two players play a game of chess, if they are both exactly equally matched in skill, in both cases it will be a coin-flip. Does that mean that chess players might as well just throw dice? No, whether the players are equally matched or not, they will prefer to play chess.