I was looking for the same information and couldn’t find it. Because this thread comes up high on the results list, here’s the answer for others who may be having the same problem. If your blog is a wordpress site, you need to go to “Users”-> “Personal Settings” and there will be a drop down list next to “Primary Blog” under which you can choose which address will function as your default blog.
HOWEVER, if your purpose is to make a particular wordpress blog or other site load when a user clicks your username, under the same menu (personal settings) scroll to “account details” to “website” an input your desired url.
Hope that helps!
