When we create username accounts the email address we use to create them becomes our unique identifier and all blog registered under that username appear in the "My Blogs" dropdown. You can change the blog linked to any username provided the blog was registered under that username in the first place. (see instructions below)
However, if you have two username accounts then you will have to log out and post as the under the second username
Instructions for linking username to blog
Dashboard -> Users -> Personal Settings -> then scroll down to Account Details. Then scroll down to where it says ‘Website‘ select the URL for your primary wordpress.com blog and then save changes.
Now every time you sign into the forum we can all click on your username and your blog will be linked to it. And every time you leave a comment on a blog your username can be likewise clicked and your blog can be instantly located.