I note this down so that next time I don’t search so long for this or hopefully someone other can use this piece of information too.
If you start the Block Editor from the Google AppInventor Designer but the “Connect to phone” button stays greyed out it maybe because NoScript is blocking this functionality.
Just allow 127.0.0.1 and everything will be fine.
(NoScript should show in Firefox that 127.0.0.1 is blocked.)