I understood theory's question to be about cards that are worse to get from the Black Market than from a supply pile. So just listing the worst cards in the game misses the point.
I'd say Minion is at least an honorable mention, since even though, yes, a single copy can be very good, it's a lot worse when you get it out of the Black Market as opposed to a supply pile. Wishing Well also qualifies, since a Black Market deck is a whole lot less likely to be conducive to Wishing Well, as you potentially have several unique cards instead of multiple copies of a small number.
My thoughts: Treasure Map, Peddler, and Fool's Gold are the big ones, in that order, followed closely by Trading Post and Chapel just because those cards are a whole lot less effective if you don't get them before turn 5.
I disagree that pulling a Fairgrounds from the Black Market is all that bad. In a Black Market deck, a single Fairgrounds is likely to be at least as good as a single Province, only cheaper and not available to your opponent. Seems like a pretty sweet deal to me.