If you can swing it, it’s really best if your blog URL and your blog title match. It will be much easier for your readers to find you. It’s quite possible, though, with the boom of new blogs out there, that the blog URL you want is taken. If that’s the case, choose a blog URL that is somehow related to your blog content or similar to its title and you’ll be just fine.
Whether you are using the URL supplied by your blog host (e.g., yourblogname.typepad.com) or a domain name (www.yourblogname.com) choose something that is easy to type and easy to remember. You don’t want your readers to have to type in wowthisisareallyfantasticunbeleivableblogyoullloveit.blogspot.com. They will probably fall asleep or find something shiny before they get to the end.
If you can’t get the same URL as the name of your blog, at least try to get something that’s related to your blog. For example, my (now defunct) personal blog was called Don’t Try This at Home. As you can imagine, that URL was not available for purchase. I ended up buying and using the domain www.donttryit.com because it was fairly close to my blog’s name. If you can swing it, though, try to get the domain and blog title to match perfectly.