I don't understand this. The commits are rewritten - do they overwrite the author to the current user, or don't they? Or is there a distinction between "author" and "committer" I'm not aware of? Which one does blame use?
Difference between author and committer in Git? -- http://stackoverflow.com/questions/11856983/why-git-authorda...
Why git AuthorDate is different from CommitDate? -- http://stackoverflow.com/questions/11856983/why-git-authorda...
What does “authored 7 days ago; committed 14 hours ago” mean on GitHub? -- http://webapps.stackexchange.com/questions/70383/what-does-a...
I found all of that by googling "author vs committer git".
The gist is that, yes, there is a distinction. If your friend, say, gives you a diff and you apply it with `git apply`, you're the committer and your friend is the author.
EDIT:
Regarding your other questions ("Which one does blame use?"):
If you run
$ man git-blame
NAME
git-blame - Show what revision and author last modified each line of a file