As a blocked user, there are no Reply buttons at all, so I cannot find any error message, Nor any explanation of why the Reply buttons do not appear.
Performance question (see comment): Should we be checking blocked status at view time? We could check via AJAX when they click into the field, but if the perf is not that bad, we might not need to worry about it.
A blocked user can't edit its page but leave a message on the page is the way to appeal from a block.
See also bug T61927: Flow: Improve the blocked-user error message