A reserved hour deserves the same respect whether its purpose is visible or private.

Time in a diary gets treated as real in proportion to how explainable it is. A meeting with three other names on it is protected; an hour marked only as busy invites questions, and the person defending it ends up justifying rest, or treatment, or something they would rather not discuss. That is a strange standard, since the hour costs the same either way. Nobody should have to disclose a reason to hold their own time.

When someone blocks time without explaining it, take it as given rather than probing. When you reserve an hour of your own, defend it with the same steadiness you would use for a meeting.