Your username can only be linked to one blog ie. your primary blog. Here’s how to link your username to your blog in your blog Dashboard -> Users -> Personal Settings -> then scroll down to Account Details. Then scroll down to where it says ‘Primary Blog‘ select the URL for your primary wordpress.com blog and then save changes. Thereafter 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.
When we register a username account here the email address we provide becomes our unique identifier. When we are logged into WordPress.com and register a blog it's registered to that username account we are logged in under and to the email address which is our unique identifier. We can register as many blogs as we wish and provided we are using the same username account then all the blogs will be registered under it and the associated email account. We can change our "display name" but that display name will appear on all blogs registered under the same username account.