From the blogs description:
"Blogs are a great place to discuss your thoughts and virtually anything goes as to what you say provided it is not offensive to other Bluelighters or isn't violating the Bluelight user agreement."
Basically, and ideas or thoughts that you have that would not justify making a thread in one of the forums, will likely fit in your blog. One of the bluelight guidelines is that we don't allow self-serving threads. These are threads that don't really generate a lot of discussion, or don't really benefit anyone other than the person making it. Those types of threads belong in your blogs.
If there is something that you just want to share with everyone, like "hey, I just did that, that , and the other thing, and thought this" then that would belong in your blog. People also use their blog to post some lyrics that they may be writing, or a short story, and others can then comment on it and you can get some feedback.
I suggest lurking over in the blogs, and check out the type of stuff that other people write.