I'm may be a little harsh but try to understand. I have a few things to suggest.

I know she mainly used this blog as a diary and she used to post a lot about her anxiety and relationship problems on it, but now the posts have gone too extreme.

Why do you get to decide what is "too extreme"?

I'm not saying that as a criticism....but have you considered that you are basically saying, "My friend is not allowed to feel and think certain things because I don't agree with them"? Your friend is using that blog as a "safe space", because she feels that she cannot open up to anyone in her real life about her feelings....because they will judge her, or tell her she's not allowed to feel that way, or that those feelings aren't acceptable or okay to have.

That is a huge problem. And clearly, she's justified in thinking that in at least some cases....because once you found out, your immediate response was, "These things are not acceptable for her to think...they're too extreme."

This blog is your friends only place where she can express feelings without being told they're not okay to have. She needs it....and if you want to confide in her that you know it's her blog, I think that is a good idea, because it's important to be honest when you're reading someone personal diary without their knowledge, and that could really make her feel violated if you don't say anything about it now. Even then, she needs to know that talking to you is a safe place where she is allowed to feel these things....and if you don't think they're okay to feel and would tell others about it, then you need to tell her you know, and then not bother her about it.

I tried contacting her on facebook but with no luck.

Give her some time to respond...did you actually tell her, "Hey, i know this is your blog, I just wanted to let you know that I know and I wanted to ask if there's anything you want to talk about"?

Is there any other way you can contact her without publicizing this secret to the whole free world?

Not too sure what to do right now. I know I could contact one of her closer friends but I don't really want to reveal the blog since it was meant to be kept secret (she deleted all of her photos on it about a year ago and kept it private at one point). I feel like it might drive her even more off the edge if word got out about the blog.

I agree. You are essentially reading someone's personal diary without their knowledge, and then taking that deeply intimate, personal information and spreading it around "for her own good" would probably result in her losing the only safe space she has.

You have to ask yourself....if the only safe space she has is one little blog, why is that? Because it sounds like that is her place to vent because she knows if anyone finds out, they'll tell others, or have her locked up and drugged "for her own good" and "because she needs help" instead of saying, "Oh wow, that's awful you feel that way, but your feelings are valid and it's okay to feel them."

The biggest thing you can do to help is probably to let her know that you can be a safe space for her too...and if you can't....if you would break confidence "for her own good" or tell her that certain things are too extreme and not okay to feel....then there isn't much you can do.