Black Market has a whole host of unique interactions, mainly through playing treasures during your action phase, but I guess that's not what you're after. So I think it's easier to look at the reasons not to buy a black market since as someone else said, there's usually something good in the market deck.
- the kingdom is weak and has few defenses, making attacks and defense via the black market more viable
- existing terminals will dominate the kingdom
- key cards need to be bought from the kingdom repeatedly (minion, hunting party)
- it's too difficult to provide actions for terminals from the black market
- black market cards will arrive in your deck too late and be played too few times to be effective
- you can't play the black market often enough to get the right cards at the right times
- you can't guarantee good spending hands when playing the black market, losing the opportunity to buy good cards