I had an old wordpress.com account, with no blog registered.
I decided to use it and create a blog, but first I wanted to change my username. So I changed it and, in “matching blog address”, I chose “create a new blog to match my new user name”.
Now, the blog with my new username exists (illogicallegria.wordpress.com); but I can’t log in it. If I log in wordpress.com with my new username, I can only “register a new blog”. Obviously, if I try to create a new one with the same address, the system says that it already exsists.