Hey,
You are interested in heavy metal music, so there's no problem in conveying this in your "fashion", in my opinion.
I mean, if you dress like a "hiphopper" and you are not listening to that kind of music - it will be incongruent with who you are.
But you are genuinely interested in metal, and probably go to metal concerts.. so why not!
My advice:
Try to find some pictures of "famous" metal musicians.
Look at how they dress, and copy them/get inspired.
That's the easy way..