All the blogs you have attached to your username are listed on your My Blogs page.
To access the dashboard of a different blog attached to your username you simply click on My Blogs and the Dashboard link below the name of appropriate blog. This takes you to that blog’s dashboard where you write posts, pages, manage comments, change your blog design, add widget etc (depending on your user role on that blog).
This means you don’t need to log out of the dashboard of one blog to work inside the dashboard of another blog!