Personally, I don't think it's a big deal. Teenagers are curious to try out different images and looks. It's true that rebellion can lead to all sorts of problems, but what could this particular hair incident lead to? I think it's relatively harmless, and I'd see it as a "healthy rebellion".
Although . . . wanting to dye her hair tells me that there may be things about herself that she doesn't like, or that she yearns for a feeling of accomplishment as if she feels like she doesn't have control over things. Choosing a hair color and dying it herself is a quick and easy way to get the feeling of control and sense of accomplishment. Sorry for the drastic comparison, but many times teens plagued with annexoria like the feeling of accomplishment because they're able to "control" what they eat.
Instead of thinking about punishments, maybe you should talk to her, woman to woman, and find out why exactly she enjoys changing her hair. Is it simply because she finds it "cool"? Or is there something about herself that she is desperatly trying to get control over, and so changing her hair is an easy concealer of the "root" problem ? (pardon the pun)
You can reinforce to her that you think she is so beautiful without the black hair and you know that others find her beautiful too. Getting so upset at the fact she made her hair darker than "you allowed" tells me that you also want to have a hand in "control". I'm not saying that parents shouldn't control their kids--but at some point it goes a bit overboard.
You know, high school is a tough time image-wise, especially for girls. You can always encourage her that in college it's going to be a lot different. I suppose you can deny her privledges to go to that concert next month; but on that day, why don't you make it a "mom and daughter day"? Treat both of you to a nice visit to a spa and salon? Pamper yourselves, get a cool makeover! You could even let HER pick out a fun new style for you from a magazine. Treat her to a professional style cut and color. Doll both yourselves up. Reinforce positive things about her and who she is. You need to help her realize and accomplish the goals she has.
Think about how you can be creating this memorable experience for years down the road by a mom and daugher spa day as an alternative to just a crumby memory of not getting to go to a concert !