What is blocking?
In roleplaying terms, blocking is ignoring or denying something your partner/partner's character said/did. Not blocking a hit in a fight, mind you, but blocking in the sense that one absolutely refuses to acknowledge the information that another writes.
To avoid unintentional blocking, please communicate before writing a scene.
Unintentional blocking may occur:
when one character assumes something about another character that is false.
when one character assumes something about another character and that is true, but the other character completely denies what the former assumed to be true.
Blocking is not deception. Blocking is not ignorance. Both of those are roleplaying conventions that can be part of a character's make-up, and may be easily forgiven.
Blocking is similar to god-modding, but is in a little field of its own. It is blatantly refusing to acknowledge something that another person does or says. This is usually an ooc decision rather than an ic decision, but sometimes it can translate into writing. This is partially why people have about pages (especially with alternate universe/event 'canon' muses), so that those that write with them gain information and do not presume anything about their muse.
Example:
Villain walks into frame. Hero shrieks, accusing villain of torturing them horribly and getting understandably upset. Villain stands there confused. Did not actually torture Hero. Just wanted to carry out his evil deeds in relative peace. Says as much to Hero. Hero continues on with his self-righteous (and totally wrong) assumptions, vows vengeance for his imagined slights that are completely irrelevant to this particular Villain.
Now, who was blocking in this scene? The answer is actually both of them. Villain is refusing to acknowledge something that Hero insists happens, but Hero is apparently misinformed and refusing to acknowledge that this is not the Villain that tortured him.
Unstoppable force meets immovable object.
Both parties are blocking the other. The scene will die and both parties will likely leave unhappy, or even angry.
Please, please, p l e a s e, communicate with your writing partners to ensure blocking doesn't happen, whether intentionally or unintentionally. And remember, scrapping or editing a scene because of blocking can be a legitimate thing that can happen. One may have to apologize, but blocking is something that can be avoided.