A noncreature permanent that turns into a creature can attack, and its {T} abilities can be activated, only if its controller has continuously controlled that permanent since the beginning of their most recent turn. It doesn't matter how long the permanent has been a creature.

Blinkmoth Nexus can be the target of its last ability after activating its middle ability. Activating it after it has blocked a creature won't remove it from combat or stop it from dealing and being dealt combat damage.