Here's a small script to allow you to do so. Download it, open it from within Radio (it'll know where it wants to be installed) then try emailing your blog. Make sure that the first line of your email message matches one of your category names. This category name should be on a line on its own in your email message. For example:
Hello to all my friends!
The category name as it appears in your email message is not case sensitive. Useful if you're a lazy typist like me.
Let me know if it works for you.