For reasons I don’t remember, I was using a different email for my forum.newsblur.com account than I was for Newsblur itself. This used to work fine, but the login flow changed so that it uses your account information from Newsblur to discover your email address. As a result, it asked me to create a new forum account when I already had one.
I fixed it by changing my Newsblur email address to match my forum account. So I’m okay now, but I think this is quite confusing.