git blame file.txt blames the version of file.txt in your working copy. If file.txt has Windows-newlines (CRLF) in the repo and you have core.autocrlf = true, then every line of file.txt will be considered different and will be reported by git blame as not yet committed.
The reason why git blame <my_branch> (or even better git blame HEAD, which works no matter what branch you’re on) works, is that it doesn’t blame the working copy version so there’s no potential for lines not yet being committed.