The git tag documentation says that you can tag either a commit or an object:
<commit>
<object>
The object that the new tag will refer to, usually a commit. Defaults to HEAD.
And indeed you can take the hash of a blob object and tag it. But I don't understand - what is it good for? what can you do with this tag?
You can tag anything to give thing a name instead of a long cryptic ID and to prevent garbage collector to remove unreferenced objects.
For example, I store my GPG public key (to verify my signed commits/tags) as a blob and tag the blob (the technics is described in Git Book, chapter Git Internals - Git References).
