I just would go with the truth because it seems sensible, I guess you could find a way of fitting in into conversation, I mean with your heavy internet usage I imagine that it is not to hard to find a link into talking about it. I have had similar issues with my blog only made harder because the majority of it these days is friends only.
I don't think the disclosure of a blog should be something to worry about t be honest even with the serial dating. You have not been dishonest with him so it should be fine, and if it is not he's not for you to be blunt. Your a beautiful, intelligent and funny person. Who could not read this blog and understand the wonder that is you *hug*.
Kat! There's no requirement saying you have to tell him *right now*. you can always reserve the right to tell him later. After all, he probably hasn't disclosed his full medical history, his torrid love affairs or his toenail picking habit - why not hold back until you spend more time together? Just because you don't tell him right now doesn't mean you can't next time, or the time after or whenever he tells you about his toenails!
I'd start with the phrase "I think I need to come clean about something," or some other version of "we need to talk" without actually saying, "we need to talk." It's so early for a phrase with such negative connotations.
Also, try and keep in mind that what you're doing will sound weird to him, so put it as gently as you can.
Ummm... how? In person, fully (and regularly) clothed. Directly.
The thing is, he will find out - and if it wer me, the creepiest part of the whole thing is not that you've been on so many dates but that he fully has an online presence as the "man from marshall's." And by telling him the truth, you're free to tell him that the experiment had nothing to do with him, he just happened to fall into it. And he then, is free to tell you whether he is comfortable with you blogging about him or continuing dating other numbered individuals. And you then, are free to decide how to respond to his response.
Worst case scenario, you'll be like those celebrity couples on "reality" shows where the one guy opts out of the show so in the "reality" the reality star female is single or just casually dating but in "real reality" she has a long term B-list actor boyfriend. And yes, I'm referring to Lauren Conrad and what's-his-name.