At what point do you disclose you have been blogging about them? I have been struggling with this myself...
That's a tough one, and I think the answer may depend partly on the size of your blog and audience and how frequently and in what amount of detail you post about your personal life. If you have a tiny blog with few readers and you post entirely for personal journaling benefit, or if you are a full time blogger who rarely divulges personal struggles or anecdotes, then an argument may be made that you don't have to tell or show your partner at all.
In general, though, I think most of us need to inform our partners about our blogs - and their role in them. It's a matter of courtesy and possibly of ethics, but your blog is probably also a unique and interesting facet or extension of your personality and life. Sharing it with someone is a big step to be sure, but hiding it can start border on secretive and disingenuous.
So when to confess your secret? The earlier the better. Most likely you've cited him or her on the blog before at least in passing before you were close, serious or comfortable enough with that person to mention it or ask permission in advance. In my case, for instance, getting to know my new bf prompted me to write about the potential pitfalls of dating a coworker.
If you tell the person fairly early on that you blog in general, you can figure out a lot by gauging their reaction. Is your partner nonchalant about it, fascinated, suspicious? This can guide you when you are deciding whether to mention the relationship on your blog or do an entire post about some personal issue. You can avoid a lot of drama if they have a really bad reaction (deleting one post without having to even show it to them if they freak out is a lot easier than having to deal with a year's worth of posts).
Giving them a bit of info up front and along the way can make the follow up conversation (when you divulge the name of your blog, your posts about them, etc) a lot easier, if and when it occurs. And it can enable you to delay that conversation until you fully trust the person and are sure the relationship has staying power (none of us want a bunch of ex-boyfriends knowing about our anonymous blogs after all). If they know you blog from the get-go then you aren't being totally secretive, even if you aren't ready to share the details.
At some point down the road when the blog comes up again - which should happen about the time you start getting tempted to write about the relationship more frequently - you could mention teasingly that you may even have mentioned him or her on your blog. His reaction to that will tell you a lot. If he smiles excitedly and digs for details you are probably ok - if he recoils in horror and you find yourself sputtering to assure him that you never used his real name, you may have an issue.
Either way you will probably want to establish some ground rules together, which may be as simple as you promising not to give any details that would give away his identity, not to slander him, or otherwise assuaging his primary concern. Again, this is earlier before you have a bunch of posts about him already up, particularly if they address sensitive issues like how you feel bad making more than he does or complaints about his habits.
My bf knew from the beginning that I blog, and I demurred when he asked the name of the blog the first time he asked. Because we were still getting to know each other I explained that it was anonymous and he accepted that - especially because it's about my personal finances, the details of which I hadn't shared with him yet. A couple of months later once he pretty much knew all about anything he'd find in there, in passing I confessed the name of the blog, I knew full well he'd immediately google it (he immediately googles everything I mention, from brands of jewelry I wear to the neighborhood where I grew up to the tuition at the high school I attended). I assumed he'd find my fairly benign and non-personal post about dating a colleague and bring it up if he had an issue.
We haven't talked about it, but he may be checking it daily for all I know (hi sweetie!). I don't plan to use real names or say anything here that I wouldn't say to him directly. Though to be fair that is easier for me because he and I both enjoy overanalyzing and divulging our opinions and perspectives to each other.
The bottom line is that this is a real part of me; if he likes me less after reading it that will be good information to have sooner rather than later.