My three entries for the Community Server 2007 Theming Extravaganza were all blog themes.
SnowScene is a fairly simple alteration of the Vanilla theme that adds a background image and some transparency to provide a Vista-ish look. It is actually a theme I started (and blogged about) well before CS 2007 was released.
SnowFlakes is based on a template I saw on OpenDesigns.org (and blogged about in April). This one was done from scratch (other than using the css template) and took quite a bit of time to complete. I really like the look of this theme and it will probably be my blog's theme soon. I believe that if I have a theme that medals at all, it will be this one. I have a site-wide theme based on SnowFlakes that is about 90% completed, but it was just going to take a ton of work to get through that final 10% and I just didn't have the time.
My final entry was just a fun one I put together a few nights ago that uses all the images from the CS team found on UnProfessionalCommunityServer.com. I thought the CS team would get a kick out of it. The little "face" icons are actually animated gifs that rotate displaying the face of each team member. This blog theme is based on the Vanilla theme as well - I find that it is a great simple, yet complete, theme that provides a good base for customization.
Creating blog themes is pretty easy, especially if you can use one of the existing themes as a spring board. I had a blast putting these together. Give it a shot, you'll be expressing your design skills in no time.