The BO templates are easily copied and pasted into a program like Dreamweaver, where you can adjust the design to your heart's content, and then just replace the BO template with your new html code. The main thing to be careful of is keeping the codes for various blog functions intact. IE where the post shows, trade link box, etc.

I believe you can do this as either a mass usage template as well as just doing it on a per site basis.