If there is no blossom sha256 for a tag, it cannot be cloned from blossom. I think it is not the expected behaviour from the user. If he makes the move from github to blossom, then removes github.com clone url from the repo announcement even, then he will be disappointed if he cannot clone old versions.

I think blossom keys should be added to each ref upon the first push to blossom to keep the functionality.