To the OP: the serious answer to your question is that there isn't an ideal Black Market deck, because the strength of a given Black Market deck depends on what's in the rest of the Kingdom.
This is because the value of Black Market is that it lets you buy pieces for your engine that aren't available in the Kingdom.
Thus, it's awesome to buy Chapel from the Black Market if there's no trashing in the main kingdom; but you'll probably ignore BM Chapel if you already have strong trashers like Remake or Donate or Junk Dealer in your deck. The same can be said for BM being the only source of +Buy, or Villages, or Cursers.
Yes, there are some cards that are terrible in the Black Market, as noted above; others, like Prince, or Outpost, or Goons, or even a more modest cards like Trade Route and Candlestick Maker can give your deck an ability that other players don't have, which can be great.