"Please at least hide or purge notifications for blocked users, or allow users to clear their own notifications."
You need to go back to that blocked user's page and, in the same menu as the block option, click the 'silence user' option. This will automatically hide notifications from the silenced user.
Please keep in mind that other users can still see comments or reblogged commentary left by this blocked user. On the bright side, after they're blocked, they won't be able to make additional comments.
They can always circumvent the block by switching to another blog, so keep this in mind if you get continued, somehow familiar, unwanted comments