It could mean nothing at all. From my own personal experience. Really you can't judge a person by what they are wearing!!! Please don't make that mistake.
I LOVE wearing black and I actually LOVE the look of *some goth clothing. Does that mean that I go by the goth beliefs or life style? - Nope. I am actually Christian. But I just REALLY REALLY like that fashion and actually think that black hair is sexy. So like I said. You really can't say that because a person dresses a certain way - that they are believing and living in any certain way.
If you really want to know about a teen-ager or any person for that matter, it is soo very important to talk to them. Get to know them. See what's on their mind and what is important to them as a person.
And remember teenagers are just discovering themselves and may have thoughts and views that WILL change over the years. So don't just write people off if they tell you things that you may not like.
Have an open mind to different tastes and fashions styles. As long as we aren't hurting ANYONE - include our selves and we aren't rebellious to authority what does it really matter. We like the way we look and we wear it...