The blob URL is created by javascript in your browser.
To download the file from a client, you have to open a websocket to download the parts of the file then decode it and concat the parts together.
See https://framagit.org/fiat-tux/hat-softwares/lufi-cli/-/blob/master/bin/lufi.js?ref_type=heads#L272 for a js implementation or https://framagit.org/setop/pylufic for a python implementation.