I think it sounds like you have been pretty practical about your daughter's internet use and despite the fact that she didn't share with you about the blog right away, she sounds responsible. All teenagers are going to be tempted to not share EVERYTHING with mom.
I don't think that means that just because she has a blog, she must be lying and switching over to MySpace everytime your back is turned.If you truly think that, you can contact either site, explain that you are the parent and your child is under 17, and get the account shut off. I know of other parents who have done so.
I consider myself to be very internet savvy- my ex, father of our son, is an internet programmer and my current fiancee is a graphic designer who works a lot online.I work for a parenting magazine and also work and blog online extensively.
Many of the concerns listed in posts below are a little unrealistic IMO, and I will tell you why.
1)Yes, ANYTHING can be hacked. But you have to look at WHAT the site is and is used for. There is a MUCH MUCH greater likelihood of a site being hacked if the hacker has something to gain financially. Or if the hack involves just setting a worm program loose to destroy computer info at random, just for the sake of destruction.
If a hacker is just trying to destroy random computers, emails are one of the most common ways to get the virus into your system. If he is looking for financial gain, a hacker is likely to try and break into shopping sites for credit card info, banks for social security numbers, etc.
In almost all blogs, there would be no way for a hacker to make money off of the hack, so there is a very very small chance of that happening.
2)Most online predators looking for child or young victims are NOT hackers. They don't know anything about computer coding or how to break into a closed or secure site.
They just log into a public site where kids or teens are known to log on, usually with a fake name, age, etc. and cruise around trying to start inappropriate friendships.
There is no 'hacking' involved at all, just lying and misrepresentation. Again, this is much much more likely to happen on Nickelodeon or MySpace or Facebook.
It is VERY UNLIKELY that your daughter's blog will attract the attention of a predator, especially if you and she and her friend and the friend's mom can agree to limit the access between yourselves. A internet predator has much easier places to find victims.
3)Some general info: If she is on Blogger.com (which I use) or a similar site, you can set it up specifically so that only people you allow to view it can see it. As long as your daughter and her friend do not open it up to the public or share the site with others, it's a pretty private, reliable way for them to 'talk' and write down their thoughts, etc.
My nine year old son wants to start a blog and I know several kids who have them and they are only open to family members, grandma and grandpa, etc.
I would allow her to keep it on the condition that you also ok it with her BFF's mom. Tell both girls that you guys will also have the password and will check it occasionally to be sure there are no additional users on it or open comments, etc.
That way it is mostly private between her and her friend, but you can keep an eye on things and have peace of mind. At 13 your daughter IS old enough to figure out how to get around you. I suggest you become more educated about the net- get her to show you how to navigate better, or take a class,etc. The more you know, the harder it would be for you to get fooled! She is going to be online more and more, in high school and college. Educate her about how to be SMART when she is online and you do the same! Good luck!