I have been dying my hair since I was 14/15. I have no idea what my natural colour is anymore (probably a lot more grey than when I started though!)
I've had it pretty much every colour under the sun.
The only thing I would say about black, is that there are far more exciting hair colours to experiment with and when you've gone black, you pretty much have to stick with it until it's grown out.
It is possible to be a Super Goth with blonde hair. If I were her, I'd be more inclined to consider a lighter blonde rather than black. Every Goth has black hair. She can be subversive within her subculture.
If it's curly, she could go red/auburn for a proper pre-raphaelite look. But then I'm guessing 13 year old goths of today aren't really into that particular goth look...
but at the end of the day, it's only hair. It grows back. And it's so much fun to play around with it.
I'm no longer Super Goth, but my son is quite proud of having a more 'alternative' mum.
Also, loving all the 'chocolate brown' type suggestions from people who clearly have never experienced the need to have hair that could not possibly be more black 