Text (VI+errata): Each land is a Swamp in addition to its other land types. [Oracle 2009/07/08]
The land can now be tapped for black mana in addition to any other abilities it already has. [Visions FAQ 1997/02/16] This works just as if the text "{Tap}: Add {B} to your mana pool" was added to each mana-producing land. [D'Angelo 1998/01/26] In other words, the land can be tapped for its own ability _or_ it can be tapped for black mana. Not both.
The ability is a land type change effect so it is ordered along with other land type change effects in the order they entered play. [DeLaney 2001/05/06]
The lands do not get the name "Swamp" in addition to their current name. [bethmo 1999/11/16] This is a REVERSAL.
Whether the land is a basic land or not is unchanged by this effect. [Duelist Magazine #17, Page 25]
If this effect is applied to a Snow-Covered land, the land is now a Snow-Covered Swamp and not just a regular Swamp. [bethmo 1997/03/23]
More than one Blanket of Night has no meaningful additional effect. [D'Angelo 1997/08/22]
Anything which counts the number of Swamps in play will not double count an actual Swamp card while Blanket of Night is in play. [D'Angelo 1999/03/11]
It can affect lands which do not produce mana. [D'Angelo 1999/11/14]
