Its not about which land it "destroys" but about the fact that they can't USE that land anymore unless they take the penalty.
It throws off the curve and tempo of any non-black deck out there...or gives you a free 2-4 damage min if they choose to push ahead.
How much damage does the almighty lighting bolt give you? Its obviously not as good for obvious reasons....but for a palty 2 mana investment....its serious damage/disruption to your opponent.
I will play this card EVERY time I get it if I'm in black.
The enchanted land loses its existing land types and any abilities printed on it. It now has the land type Swamp and has the ability to tap to add {B} to its controller's mana pool. Contaminated Ground doesn't change the enchanted land's name or whether it's legendary, basic, or snow.
If it said "in addition to..." it would keep its current land types. See Prismatic Omen .
