There are as many genres as there are individual blogs.
Just kidding.
I would say that blogs can be divided into genres by their medium and the number of users as well as their content. The word "blog" itself signifies a rather broad spectrum of online media, from the traditional single-user text-based blogs (perhaps with a few images) to collaborative video blogs. Having said that, these are the blog "genres" that I can think of, just off the top of my head:
Entertainment blogs
Personal blogs
Political blogs
As previously stated, sometimes the medium itself can be thought of as the blog's genre:
Music blogs
Video blogs
Obviously, these genres can be combined into hybrid blogs, or divided further into an infinite number of sub-genres. Furthermore, the possibility of multiple authors adds a whole new layer of possible genres.
I have left out one obvious genre, since it seems somewhat redundant: News blogs. All blogs are essentially news sources, albeit sometimes dealing with matters of questionable public interest.
