Blood Moon reads: "Nonbasic lands are Mountains." The key word is nonbasic. A basic Mountain is a basic land, so it falls entirely outside Blood Moon's effect. It continues to function exactly as printed — a basic land with the Mountain subtype that produces one red mana.
Under CR 305.6, basic lands are defined as lands with the supertype 'Basic.' Blood Moon's continuous effect (a type-changing effect under CR layer 4, and an ability-removing effect under CR layer 6) applies only to lands that do not have the Basic supertype. Your Mountain already is a Mountain with its normal mana ability, so the enchantment changes nothing about it.
It's also worth noting that even if Blood Moon somehow tried to apply to a basic land, the basic land's printed text and subtypes are part of its copiable characteristics and would interact under the layer system — but the card's own wording simply excludes basics entirely, so that situation never arises.
Concrete example: You control Blood Moon, a basic Mountain, and a Stomping Ground (a nonbasic land). Your Stomping Ground loses its Forest and Mountain subtypes and all its printed abilities, becoming a Mountain that taps for one red mana. Your basic Mountain is completely unaffected and still taps for one red mana as normal.
Unofficial fan resource — not affiliated with or endorsed by Wizards of the Coast. Answers are AI-generated estimates grounded in the Comprehensive Rules and are not a substitute for an official judge. Verify anything match-critical.