Is the default mutual? I've never seen it done otherwise that I know of, and inertia is a powerful force.
Why would you expect to see it? If it is not mutual, there's no outward sign of it.
James blocks Mark in the mutual way, then James can't see Mark, and Mark can't see James. Mark might notice and comment on how he cannot see James.
If James blocks Mark in the non-mutual way, then James can't see Mark, but Everyone can still see James. Mark won't comment on it, because they still see James just as they did before.