Got a VERY alpha software Blossom server impl going on here, written in rust: https://github.com/0xtrr/cherry-server. It probably has a bunch of bugs and is not tested well enough so nobody should use it yet. It currently has pubkey and mimetype filters and works behind nginx. Code is a mess and I've probably pulled in more deps than  needed. Depends on a sqlite3 DB to keep track of file references and blob descriptors.