Yes exactly, and it's skill to be able to quantify the probabilities and formulate your decision based on that (even if "quantifying" it is not done with a calculator, but with your intuition).
For example (using the example above)...
Witch/Montebank are in the black market deck.
I probably have a 50% chance to win this game without black market.
If I buy a black market and get an early witch/montebank, I have a 70% chance to win.
If I buy a black market and get an mid witch/montebank, I have a 55% chance to win.
If I buy a black market and get an late (or no) witch/montebank, I have a 30% chance to win.
Then you guy-instinct the probabilities of each of those events happening. Of course, you could actually calculate it, if your opponent let you get out a calculator/spreadsheet, but likely you are going to have to gut-feel-it in a game.
Then you gut-feel the "randomness" factor, and decide if you are comfortable. I.e. making this decision will likely make the game more luck based than not making it. Do you feel you can outplay your opponent more if you play a less luck-based strategy? Do you feel you may get outplayed more if you don't?
For example, lets suppose the decision came down to 50-50, you were just as likely to win using this strategy as not using it, but using it means the chance of you winning is more based on luck (getting the W/M or not). If I was against the #1 ranked isotropic player, I'd likely go with it and roll the dice, since I'd likely get outplayed if I went for a less-luck based, but equally strong strategy. If I were against a weaker player, I would try to outplay him instead.
This is where skill/strategy comes into play and dances a tango with "luck". Good players will be able to assess this "luck" and make an informed strategical decision, and they'll make the correct decision more often than a lesser skilled opponent.
Anytime you introduce a new decision, you introduce the opportunity for one player to outplay the other - even if that new decision has a lot of "luck" associated with it.