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!
Try to find some pictures of "famous" metal musicians.
Look at how they dress, and copy them/get inspired.
That's the easy way..