Umbrel by default have in bitcoin.conf file the SPV filters activated
So in Blixt you only have to connect through Tor to your node onion address
The onion address for BIP157 P2P endpoint, is NOT your regular Umbrel onion address!
You go to Connect wallet - Wasabi for example and there you have the onion address for your SPV server (BIP157).
Copy without port 8333 and put it into your Blixt wallet settings. Before that activate Tor connection.
Update: for the time being, through Tor is not supported, due to incomatibility of LND with v3 onion
But just tested through the LAN, just used node local IP:8333 and worked.