I found a solution.
After all hints from BlueMail support failed, I created an app password in my Gmail security account and then used the manual option to enter Gmail’s parameters, with the new app password. I’m now connected. It seems BlueMail for Linux is somewhat considered “insecure” enough by Google to allow it. No such an issue with the Android version at all.
I have used many email apps in Linux over the past two decades, from Balsa and Sylpheed to Evolution and Thunderbird. But I consider BlueMail the best email app for Linux right now, even though it has its issues. The rpm version did not even launch in Silverblue. But it has a modern look, somewhat similar to Geary, with several other unique features I love to use.