Some ways to navigate through 'git blame' over time in GNU Emacs

My favorite tool for this is C-x v h (vc-region-history). It's like C-x v l but only shows log entries that touched the current region.

Tangentially related, you can easily see every commit that has changed a language-level function (see git's regexes here) using M-x magit-log-trace-definition or just t from the magit-file-dispatch menu.