This your secondary blog. You cannot change its URL, but you can make it your primary blog by changing your username to…
First: I don’t understand this. This is my only blog and it is listed under my username.
Second, I have a custom domain: gebloom.com. Are post URLs supposed to be listed like this? https://write.as/gebloom/<slug> rather than https://gebloom.com/<slug>
I see that message too, I get the sense from this and my own question that the multi-blog stuff might be an afterthought and not really ready for prime time unfortunately.
Just looked at your account and I’m not seeing the same thing… Could you try logging out and back in? I think there’s a bug where if you log in and accidentally add a space before or after your username, you’ll get logged in correctly but it will cause this strange behavior.
Yes, you’ll see those write.as/<username> links while you’re logged in, but readers will always see your custom domain (and be redirected there if they go to your write.as URL).