The Dross Pits

The trade is written into the sequencing: enters tapped, taps for one black, then cashes itself in for a card once black decks run out of gas. That last clause is the whole point, and it is the payoff black wants more than any other color's version of the effect: a spare card is never a dead draw in a deck built around attrition. The cost structure keeps it in check. Coming into play tapped means it never accelerates you, and the draw asks for one black plus one generic on top of the sacrifice, so the land is a resource you spend late, not a tempo tool you deploy early. The one distinction worth noting is that it produces black without carrying the Swamp type, so anything keyed to basic land types (fetches, tutors, the odd Swampwalk) reads right past it. Otherwise it sits in the deck as a plain black source until the point where flooding out would otherwise lose you the game. Then it converts a would-be dead land into velocity. This is the black entry in a cycle of "sphere" lands that each fix one color and later sacrifice for a color-appropriate payoff, part of a longer lineage of lands that offer a mana ability up front and a sacrifice ability later. Black is the color that most wants the "draw a card" version because refueling is its identity. Turning your least useful late draw into your most useful one is exactly the job black asks its lands to do.