Blocks don't have a 'view' capability (usually anyway) so you can't do that. It would settle it if you told us which block?
There are a few blocks with the capability moodle/block:view so you could make a new role, add it to the block context and set it to allow for that role and prevent for other roles and then assign it to the one particular person. But you need also to think where the block will be visible because users have different roles depending on where they are on the site and so visibility may change. For some inspiration, take a look at the relevant questions in Blocks FAQ
Naturally, the random sample I looked at didn't
I think that's another one for the 'consistently inconsistent' pile