Each land is a Swamp in addition to its other land types.
10/4/2004 The land can now be tapped for black mana in addition to any other abilities it already has. This works just as if the text “: Add ” was added to each mana-producing land. In other words, the land can be tapped for its own ability _or_ it can be tapped for black mana. Not both.
10/4/2004 Whether the land is a basic land or not is unchanged by this effect.
10/4/2004 More than one Blanket of Night has no meaningful additional effect.
10/4/2004 Anything which counts the number of Swamps on the battlefield will not double count an actual Swamp card while Blanket of Night is on the battlefield.
10/4/2004 It can affect lands which do not produce mana.
10/4/2004 The lands do not get the name “Swamp” in addition to their current name. This is a reversal of older rulings.
10/4/2004 The ability is a land type change effect so it is ordered along with other land type change effects in the order they entered the battlefield.
10/15/2006 If this effect is applied to a snow land, the land is now a snow Swamp and not just a regular Swamp.