Been a long time UG,
I just recently auditioned for this great local black metal band as guitarist. They have their old songs which I am learning right now But I'm a little concerned. As in all Black metal there is a lot of two string Constant 16ths at a breakneck pace. I normally play thrash and Death metal so I'm quite used to picking fast, Per single string though. These double string Riffs are starting to cramp up my arm and I kno it's quite common in black metal for dudes to get tendinitis or carpal tunnel because of it. I'm not sacrificing my wrists and arms just to play black metal. So my question is what way should I start tackling these riffs without strain or injury? Any picking techniques etc. Is much appreciated
Only advice I can give on on this regard is warm up well before playing this stuff. Your muscles have to be warm and flexible when doing that stuff.

Ben Ash from Carcass also has couple of tips here at 8:03. Good posture and good picks to minimise the impact from the strings basically.

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don't twist your picking hand into weird positions when you do the tremolo picking. i find that the closer to neutral i leave my hand, the more endurance i have. it feels like you can pick faster if you lock your bent wrist and rotate, but it tires the hand quickly (and is probably not healthy either).

keeping the hand in neutral also gives you more picking range so you are able to tremolo-pick chords better.

and obviously, no anchoring.
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+1 to all the advice above.

I'm probably guilty of not warming up before blazing away, but it does help in a big way.

However, I think what's even more important is to use only wrist movements and concentrate on making your motions only large enough to get the job done. It should be a relaxed set of motions, but small and controlled at the same time. If there's any pain, stop what you're doing.

It's one of those areas in guitar where you really just have to put in your time and build up endurance slowly. And even though I've been playing black metal for a long time, I still get tired when playing certain passages. (The worst is one of my own songs...8 minutes of straight tremelo picking, galloping, and more tremelo picking over chords).
i'm by no means greater than anyone else, but the one thing i know is that you should NEVER be playing anything that causes you pain or severe discomfort, since that will obviously do great damage down the road, i had to completely overhaul my own technique when i started playing Black Metal music.
the true mission for realizing one's own style is discovering how to do everything you need with as little effort as possible, which may at first require you to slow down tremendously to really think through how to achieve proper motion. i know it did for me, and still does everyday! all of my inconsistencies in playing come shining through when playing music like Black Metal, so i have to take everything very slowly/carefully at first with every new riff i write, and it's important to do so, not only for learning the part, but for really getting to 'know' what yr doing and trying to accomplish with each measure.
i too have some issues from time to time with tremolo picking on chords, depending on the location on the neck. i realize this post is like a year old, but if anyone else out there has any advice i'm all ears...
since the OP is a veteran of Thrash and Death, i'm sure he's done fine, but if yr still on here, i'd love to hear how yr progress went. i'm new to forums, but i love to learn new techniques for guitar playing especially in this style of music! cheers to anyone who read this ridiculously long reply...