How would a blind person know they are boarding a coach that is or isnt the quiet carriage?A rather manipulatively worded response, if you ask me. It was quite clear the post was not meant with malice. It's a very reasonable question to ask why so-called Quiet Carriages are still plagued with announcement spam.
Even so, of course they'd be allowed to sit in quiet carriages. But if they benefit from plenty of loud and clear auto-announcements, they'd likely be better off choosing to sit in one of the other carriages. It's their prerogative.
An announcement saying this is the quiet coach would surely annoy said people who don't want any in the quiet coach