My spiel on this idea is that it can't be balanced if the penalty is a one-time penalty. So if it's a card that's worth -1 VP, that's a one-time penalty. You lose a point, and thereafter it's a Gold equivalent. It doesn't scale right, because different boards and indeed different playthroughs of the same board, vary SO wildly in terms of how much a single VP is worth. It's not just a matter of "sometimes it's strong, sometimes it's weak," it's a matter of "usually it's overpowered or underpowered and only rarely just right by accident."
On the other hand, if the penalty is continuing, you can achieve some sort of rough parallel between the amount of benefit you get vs. the amount of drawback. Buy an Embassy early, or buy one late -- either way, your opponents get the benefit of a Silver a roughly proportional amount of the time. There's still room for strategy, because you can observe that buying an Embassy when you have a fat deck and your opponent has a slow deck will skew the balance in your favor.
Similarly, the penalties of Bishop, Vault, and Council Room all apply each time you play the card, not a flat penalty at the outset. And again you can use strategy to swing the scales in your favor, like using Bishop after your opponent has already trashed down, or using Council Rooms when you might draw a Militia to play afterwards.
So the flat VP hit on Blood Money doesn't work. Gaining a Curse when you gain Blood Money might, because the Curse in your deck will be a continuing detriment, but the fact that the Curses can run out so easily is problematic.
I really believe the best incarnation of the idea is Cache. It's Coppers, not Curses, which are dramatically less likely to run out. And the balance is better than most Blood Money fan cards, too, which overdiscount the card. For two Coppers you only get a $1 discount on Gold. That $5-$6 threshold is so important, though, that's all the discount you need. And you can still pull off fancy tricks like using Watchtower to trash the Coppers, Trader to turn them into Silvers, and playing Big Money or an alt-VP rush where the Coppers aren't so bad.
Directly penalizing the score is, I think, a lot less interesting. It skips over interfering with the machine you use to get points and just manipulates the points directly instead. Yawn. The heart of the game is the deck, not the scoreboard. Nonetheless, I think a score-manipulating Blood Money ought to be doable if the penalty happens on-play, in keeping with the principles of balance I started this post talking about. "When you play this, gain a Curse token." A Curse token would be like a VP token but worth -1 point instead of 1. Something along those lines can probably work.