In GitHub URL’s: what is the difference between a tree and a blob?

GitHub’s website currently seems to be:

• Using blob for files, and tree for directories, in URLs;
• Redirecting browsers which request file URLs containing tree to contain blob instead; and
• Redirecting browsers which request directory URLs containing blob to URLs containing tree instead.

It’s possible that GitHub’s website, at the time you asked the question, was only rewriting file URLs between tree and blob, instead of properly redirecting them. (Rewriting and redirecting URLs are activities of web servers.) If so, then the change wouldn’t appear in your browser’s address bar. But maybe you didn’t mean that.