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

Blood Moon has a static ability that continuously modifies the characteristics of all nonbasic lands, making them Mountains. Static abilities apply as long as the permanent is on the battlefield — they don't 'trigger' on entry and don't care about the tap state of affected permanents. (CR 604.1, 604.2)

When Blood Moon enters the battlefield, a layer-system update occurs immediately. Under CR 613, type-changing effects are applied in layer 4, and ability-removing effects in layer 6. All nonbasic lands lose their existing land types and any abilities granted by those types, and gain the Mountain land type (and thus the ability to tap for red mana) — whether they are tapped or untapped at that moment.

Being tapped simply means the land is in the tapped state; it has no bearing on which characteristic-defining or continuous effects apply to it. Tapped permanents are still affected by continuous effects just like untapped ones. (CR 302.6 for reference to permanent states; CR 604.1 for static ability application)

Concrete example: Your opponent controls a tapped Breeding Pool (an Island/Forest nonbasic) that they used for mana this turn. You cast Blood Moon. The Breeding Pool immediately becomes a tapped Mountain — it loses Island and Forest subtypes and all associated abilities. When it untaps next turn, it will tap only for red mana, not blue or green.

One nuance: lands that were already tapped do not untap when Blood Moon enters; they simply have their types and abilities changed while remaining tapped. Their tap status is unaffected by the type change.