I still want to start a character blog though! Are there any solid guidelines on doing that?
There are a couple of things too keep in mind, particularly if you're writting an IC blog.
1) You're going to have enemies. They are going to look for ways to 'win' against you. They will use anything you post, regardless of how fucking obvious you make it that it's private and not knowable IC, to attack you. Get used to it, or never post anything particularly interesting. Or stick to stuff that is simultaneously interesting without showing off anything private or any character faults, etc.
2) Even people who aren't trying to use it against you will still use things posted publicly as IC knowledge regardless of whether they should. If your RP sphere is strictly limited to people who won't do this, it's not such a big issue, but if you're on IGS, in the Summit, anywhere else where people congregate, you're going to deal with people who read your blog and then try to use that info IC.
Bear in mind that even with people who won't actively use such info IC, knowing it OOC may color their behavior when encountering that info IC. Maybe they won't be as surprised by a secret because they already knew, etc. Or, over time, they'll forget that something they know OOC isn't supposed to be IC - they'll remember learning it, but forget that it was from the blog, and now the character will accidentally refer to it IC.
Fuckin' messy shit right there.
Another option is to write the blog and not publish it publicly, share it with only people who ask to see it and you can talk with about what knowledge their character has access to and so on.
Of course, paying attention to all of the above can make things boring. It's a trade-off.