Serious answer:
Wear whatever you want. Most people will be wearing black band shirts and jeans. You'll also see some battle jackets. No one will give you shit for turning up in work clothes or whatever, but if you're nervous about fitting in you can't go wrong with jeans and a band shirt or other dark t-shirt.
I'm assuming you're not going to be stage-diving or crowd-surfing. If you see a space open up in the crowd, it's about to become a mosh pit, although I don't know how much moshing there'll be at a black metal show. Avoid going into a mosh pit until you've taken some time to observe what other people are doing.
If you want to be on the barrier, go there as soon as the doors open and stay there. Don't be that dick who tries to fight his way there in the middle of the set. I recommend not being on the barrier your first time, but it's your call.
If you want good sound, look for the sound guy. He'll probably be about 50-100 feet back from the center of the stage in front of a big console. Standing in front of that you'll get the best sound in the room.
If you want to sit at any point you'll need to grab a spot immediately and keep it.
If you drink, don't get sloppy drunk and don't spill on anyone. Feel free to go nuts during your favorite parts, but try to be aware of others - if you're not up front, it's rude to be smacking someone with your hair every time you headbang. If someone bumps into you, don't be a tough guy. If you're anywhere near the pit, you might get slammed into/kicked/punched/etc. Push them back into the pit but don't take a swing or anything, remember it's all in good fun.
If anyone at any point asks you to "make some noise," feel completely free not to. Personal, my throat tends to get sore during concerts from all the shouting and by the end I do not want to "make some noise."
My own personal do: show up for the openers. After their sets they'll probably be hanging out around the merch booth. If you like them, buy their shit and tell them you dug it. Those guys work hard and a lot of people don't even bother showing up to hear them. They can use the props a lot more than some international metal legends.