Does Blood Moon affect lands that have already been tapped for their original mana type before it resolved?

Blood Moon is a continuous enchantment that modifies the characteristics of nonbasic lands, making them Mountains (with the mana ability of Mountains: add one red mana). This effect applies from the moment Blood Moon resolves and remains in effect as long as it stays on the battlefield (CR 613.1).

Mana that was already produced and placed into a player's mana pool before Blood Moon resolved is not retroactively changed. Mana abilities are activated and their effects — adding mana — happen immediately (CR 605.3a). Once mana is in the mana pool, it exists independently of the permanent that produced it (CR 106.4).

However, any attempt to tap those same nonbasic lands after Blood Moon has resolved will only produce red mana, because Blood Moon's layer-4 type-changing and layer-6 ability-removing effects now apply to them (CR 613.1b, 613.1e). Their original mana abilities have been replaced by the basic Mountain ability.

Concrete example: Your opponent controls a Breeding Pool. They tap it for green mana, then you flash in Blood Moon in response to a spell they're casting. The green mana already in their mana pool is still green — they can spend it normally. But if they try to tap the Breeding Pool again later, it now only produces red mana.

In summary: Blood Moon affects what nonbasic lands can do going forward, not what they have already done. Mana already floating is safe from the enchantment's effect.