Subtask of T317148: Structured mentor list: Blocked mentors [tracking], which has the generic discussion on blocks and mentorship.
In T317148, we decided to mark sitewide blocked users as away automatically. If they get unblocked (or the block expires), their Mentor status should revert back to what it was before (including possible expirations of manually set "Away" status).
Given a Mentor receives a sitewide block,
Then their Mentor status is automatically set to "Away". The mentor cannot mark as active via Special:MentorDashboard.
Given a Mentor receives a sitewide block and their Mentor status is set to "Away",
Then if/when the block is over, they have the option to set their Mentor status as "Active" (and they will have the same mentees as before the block).