A blocked account can still occupy an unblocked part of the mind.

Blocking removes someone from your screen and not from your head. The mental space they occupy, the imagined conversations, the checking impulse, all of that continues, and it can be confusing to have taken decisive action and still feel occupied by the person.

Treat the block as the practical half and expect the other half to take time. Occupy the space rather than fighting it, and be patient with how long the imagined arguments continue. The gap between having stopped contact and having stopped thinking about someone is normal and it does close.