Trying to force create blog same as user name..

How can I have new sign-ups automatically get one blog with their username as the url?

The whole idea of signing up on my site is to create a blog. I thought I could simply have the checkbox checked and hidden, but then the url and blog title part of the form will not display.

I used to have it set up on the 2 page sign-up process, where the check box was hidden and the url field in the second page was not editable. With the single page process I can no longer do that. Any ideas, anybody? Thanks