Does Blood Moon affect non-basic lands that are already tapped for mana when it enters?

When Blood Moon enters the battlefield, it immediately applies a continuous effect that changes all nonbasic lands so they have the single ability "T: Add R" and lose all other land types and abilities (CR 305.7, CR 613.1b). This is a layer 4 (type-changing) and layer 6 (ability-removing/adding) effect that applies instantly.

However, mana that was already produced and is sitting in a player's mana pool before Blood Moon entered is simply mana — it has already been generated and exists independently (CR 106.4). Blood Moon does not retroactively change or remove mana already in a pool. The game does not 'undo' the tap or the mana production.

It's also worth noting that a tapped land is simply tapped — its status as tapped or untapped is separate from Blood Moon's type-altering effect. Blood Moon does not untap lands when it enters, nor does it affect their tapped/untapped status (CR 305.6). The land will still be tapped and Blood Moon's effect will apply to it when it next becomes relevant (e.g., if it somehow became untapped).

Concrete example: Your opponent has a Breeding Pool (a nonbasic land). They tap it and add GU to their mana pool, then you cast Blood Moon. The GU mana remains in their pool and they may spend it normally. The Breeding Pool is now a tapped Mountain with no other abilities, but the mana already produced is unaffected.

In summary, Blood Moon only governs what a nonbasic land can do going forward. It cannot reach back in time to invalidate mana that was already added to the mana pool before it entered (CR 106.4, CR 613.1).