MTG Treachery

{b} is the blood symbol. It represents one blood counter.
Blood counters are a kind of counter that a player may have. They’re not associated with specific permanents. (Other kinds of counters that players may have include poison, energy and experience.)
Keep careful track of how many blood counters the Leader player has. You may do so by keeping a running count on paper, by using a die on that card, or by any other clear and mutually agreeable method.

If an effect says you get one or more {b}, you get that many blood counters. To pay one or more {b}, you lose that many blood counters. Any effects that interact with counters a player gets, has, or loses can interact with blood counters.
Blood counters aren’t mana. They don’t go away as steps, phases, and turns end, and effects that add mana "of any type" to your mana pool can’t give you blood counters.
You can’t pay more blood counters than you have.