When you start BlogThing for the first time, you'll see the following:
Enter your blog's web address here, and click Next.
The page will expand to offer you a username and password field. This is the same username and password you would enter on your blog to add posts and do other tasks.
Then when you're finished, click Next.
There is another possible use for the username and password: if you're someone that's password-protected their site.
This is distinct from the username and password you use to log in and create posts. It's a case where the entire site is password protected, for example, if you've got personal material on there that you don't want the world to know about.
In this case, you'll be prompted for the username and password twice: Once for the site and once for posting.