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

Blood Moon has a static ability that creates a continuous effect: 'Nonbasic lands are Mountains.' This effect applies to all non-basic lands on the battlefield the moment Blood Moon enters, regardless of their current tapped or untapped status. Tapping a permanent is simply a state it can be in; it doesn't shield the permanent from layer-based continuous effects (CR 613.1).

Continuous effects from static abilities are applied in a layered system (CR 613). Blood Moon's effect operates in Layer 4 (type-changing effects), removing all other land types and abilities from non-basic lands and making them Mountains with the basic Mountain mana ability. This happens instantly and universally, affecting every non-basic land regardless of its state.

Importantly, changing a land's type and removing its abilities does not untap it or cause any triggered effect related to tapping. If a land was tapped before Blood Moon entered, it remains tapped, just as a Mountain. It simply loses whatever abilities it previously had — it cannot be 'used' for those old abilities after the fact (CR 606.2 covers activated abilities ceasing to function when conditions change).

Concrete example: Your opponent taps their Breeding Pool for blue mana in response to Blood Moon entering. The mana is already in their mana pool and is fine. But if a second Breeding Pool was already tapped (say, for a previous action) when Blood Moon resolves, it is now just a tapped Mountain. Its {G/U} tap ability is gone; when it untaps, it will only produce red mana.

So Blood Moon immediately reshapes all non-basic lands — tapped or not. A tapped non-basic land loses its abilities the moment Blood Moon's effect is applied, meaning it cannot produce its old mana type or activate its old abilities once it untaps.