What happens when two global enchantments affect the same type of permanent (e.g.lands)
Non-basic lands will be only mountains. Basic lands are their original type and also swamps.
Lands are basic if and only if they have the &quot;Basic&quot; type. All other lands are non-basic. Therefore, Blanket of Night does not turn lands into non-basic lands, because it only adds the &quot;swamp&quot; subtype and the &quot;T: Add B&quot; mana ability to lands.

205.4c Any land with the supertype “basic” is a basic land. Any land that doesn’t have this supertype is a nonbasic land, even if it has a basic land type.

If a land was basic before, it still is - for example a Forest with types &quot;Basic Land - Forest&quot; is now a &quot;Basic Land - Forest Swamp&quot;, with the ability to produce 1 green or 1 black mana.
If a land was a non-basic land, it still is. Blanket again adds the swamp type. For example Gemstone Mine will be a &quot;Land - Swamp&quot;, and it will gain the &quot;T: Add B&quot; ability in addition to its printed abilities.
Blood Moon then replaces all of that land's types with &quot;Mountain&quot; and all its abilities with &quot;T: Add R&quot;, including the swamp gained by Blanket, so the resulting lands will have the types &quot;Land - Mountain&quot; and the only ability &quot;T: Add R&quot;.
Note that for non-basic lands, the order in which you play the Blanket and Blood Moon matters, because of the layer system.


Interaction of Continuous Effects

613.1d Layer 4: Type-changing effects are applied. These include effects that change an object’s card type, subtype, and/or supertype.
613.2. Within layers 1–6, apply effects from characteristic-defining abilities first (see rule 604.3), then all other effects in timestamp order (see rule 613.6).

Timestamp order is, generally, the order in which an object or effect entered the game.
If you cast the enchantments in the order you described, Blanket first then Blood Moon, the result will be as described above.
In the reverse order, Blood Moon first turns all non-basic lands into non-basic mountains, then Blanket adds the swamp type and ability to those lands. These lands will then have the types &quot;Land - Mountain Swamp&quot; with respective mana abilities instead of just &quot;Land - Mountain&quot;.