Text (UD+errata): Until end of turn, whenever a player taps a Swamp for mana, that player adds {B} to his or her mana pool (in addition to the mana the land produces). [Oracle 2009/02/01]
Affects lands tapped for rest of turn, not just swamps in play at the time it resolves. [Urza's Destiny FAQ 1999/05/25] This is because it affects players and not the lands themselves. [DeLaney 1999/06/13]
Affects lands that count as a swamp. For example, if you tap Badlands for {R}, it will produce an additional {B}. [Urza's Destiny FAQ 1999/05/25]
