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

Blood Moon creates a continuous effect that changes the subtype and abilities of nonbasic lands to make them Mountains (tap for red mana). This effect only applies after Blood Moon has resolved and is on the battlefield — it does not reach back in time to affect anything that happened before it entered (CR 613.1, 614.2).

Mana produced by a land is placed into a player's mana pool at the time the land's ability resolves (CR 106.4). Once that mana is in the mana pool, it exists independently of the land that produced it. Changing the land's characteristics afterward has no effect on mana already floating in a player's pool.

Additionally, a tapped land is simply a land in the tapped state — the act of tapping and producing mana is already complete. Blood Moon's static ability modifies the land's printed text going forward, but it cannot retroactively alter a past game action (CR 101.1 general causality; CR 613.1 continuous effects apply in the present).

Concrete example: Your opponent controls a Breeding Pool. In response to your Blood Moon, they tap it to add {G}{U} to their mana pool. Blood Moon then resolves. The {G}{U} mana is still available to spend normally — Blood Moon did not affect the already-produced mana. Going forward, their Breeding Pool is now a Mountain and will only tap for {R}.

So: any mana produced before Blood Moon resolves is already done and is completely unaffected. Only future uses of that land (and other nonbasic lands) fall under Blood Moon's continuous effect.