Blossom servers are normal webservers with prefifned endpoints. Files are stored and accessed using their sha256 hash (as filename).
If you then upload a picture and reference it in you nostr post for example. People can fetch that image not just from the server you referenced but from any other blossom server that holds a copy of the file.
And because the filename is the hash you can guarantee that the file hasn't been meddled with (malware) by a server you get it from.
So no, BitTorrent is not involved and it's not a peer-to-peer system.