I read a lot of sex blogs (and have just started a 'dating' blog with my GBF!!) and I think the general etiquette is thus: Sleep with them a few times and blog about it in general terms. Reveal existence of blog. Ask permission to write about said person on blog. Offer to show person blog.
Hopefully what should happen is that the person will be open-minded enough not to mind as long as their anonymity is preserved, and all will be well. But it raises a lot of questions.
Since a blog is a place for you to write about your feelings and experiences, is your writing going to be coloured by knowing that your partner is reading the blog? Would you subconsciously censor? And in that case, is it better to keep it a secret altogether, knowing that you're not doing any harm?
Personally, if a guy I was seeing told me he had a sex blog, I would ask for the link but not look. I couldn't read about a bf's past sexual experiences, and I'm not sure I'd want to know what he was saying about me. But at the same time, I'd want to know that I could look if I wanted. Does that even make any sense?
I wouldn't be put off at all, I'd be quite pleased that he was a writer first of all, and secondly, that he has a similar level of interest in sex to me! In fact, a boy with a sex blog is EXACTLY what I need.
Content-wise, you're right, imelda - I think if the content was particularly fetishy I might be a little weirded out, but I'd be more worried if it was disrespectful, you know? If it showed a hint of misogyny or disrespect, or had a 'player' theme.