If someone is saying something nasty about me on a blog, can I sue them?

You can definitely sue them; whether you will win is another matter. Bloggers, like other publishers, are protected by the First Amendment. So they can say anything they want as long as they are not maliciously misrepresenting the facts. If they are doing that, they can be sued for libel just like a newspaper can. But if someone is calling you a jerk, that will likely be deemed an opinion, and opinions (unlike lies) are protected. Also, under federal law, the host of a blog is protected from comments posted by third parties. So if someone maliciously lies about you on a blog, you’d have to go after the poster, not the host of the blog.