Blossom does not have anything in the spec right now for processing blobs. although its something I've been thinking about adding.

Optimizing media and removing exif metadata is a trusted operation and isn't deterministic. so it has to be performed by either the client or a trusted server.

Ideally a user would select a specific server that they trusted to "process" their media, and return the new blob / sha256. then the user (or the server) could upload that to multiple other blossom servers for resilience