Blood Poisoning

Enchantment
At the beginning of your upkeep, you get one poison counter.
So if poison decks are going to have something to do with the counters besides just killing your opponent, they need to be easy to get - if they're hard to get, you want all the work you put into generating them to directly advance your goal of winning the game, not muck about doing cute but ineffective tricks.
This card would be the staple four-of in poison decks (and probably should be common if poison would impact limited). It's VERY clearly in-flavor for black to hurt itself towards some goal.