Of course you can set your blog so that no one can see it.
I'm also not so sure about using a blog to write bits of a memoir. Although my previous blogs have contributed to the piece I'm writing about playing WoW, I'm not so sure the structure of the blog is a good one if you want a longer, book-length work, to have an uninterrupted narrative arc.
I still think blogs are useful for creative writing prompts, especially shortish pieces, and for exploring new ideas. I know a lot of people use blogs as intellectual journals, places to reflect on all kinds of things, but I'm mostly interested in seeing what kind of role they might have in the imaginative life, what role they might play in helping us to write more and better and be disciplined to write more and better.