Does Blood Moon affect nonbasic lands that are already tapped for mana before it resolves?

Blood Moon's effect is a continuous effect that modifies the characteristics of nonbasic lands — specifically, it removes all other land subtypes and abilities, replacing them with the Mountain subtype and the ability to tap for red mana (CR 305.7, 613.1). This effect applies from the moment Blood Moon resolves and remains in place as long as Blood Moon is on the battlefield.

Mana that was already added to a player's mana pool before Blood Moon resolved is completely unaffected. Once mana is produced and placed in a player's mana pool, it exists independently of the permanent that produced it (CR 106.4). Blood Moon cannot reach back in time to change mana that has already been generated.

Additionally, even if a land is tapped when Blood Moon enters the battlefield, Blood Moon's continuous effect still applies to that land — it just won't matter for mana purposes until the land untaps and is used again. The land immediately loses its old abilities and gains the Mountain ability, but since it's already tapped, the player simply uses it as a Mountain on future turns.

Concrete example: Your opponent controls a Breeding Pool. On their upkeep, they tap it for blue mana. You cast Blood Moon in response — the blue mana is already in their pool and stays blue. However, Breeding Pool is now a Mountain for all future purposes, and when it untaps, it can only tap for red mana.

This is grounded in CR 106.4 (mana in the pool is independent of its source) and CR 613.1 (continuous effects apply from the moment they exist, not retroactively).