Even if the average converted mana cost of your black aggressive deck is, say, 2.4 - which is a conservative estimate - you have to remember that Dark Confidant will flip up lands a lot of the time. Assuming 23 spells and 17 lands, the average damage you'll take per turn is (23 * 2.4) / 40 = 1.38 damage - i.e. a hair over 4 damage over the next three turns - which is well within reason for the snowballing card advantage he provides. He's a one-toughness dude, too, so whenever you feel like his job is complete, you can start sending him into the red zone to meet his maker.