Triple posting is a no-no.

Also, Shred =/= black metal.
Last edited by AA00P at Dec 15, 2008,
Black metal and shred? When did that happen?

But seriously, I assume you mean not just all power chords over and over? The riffs for Guilded C*** are kinda....not really power chords. Trem picking and all. I don't believe Meshuggah are black metal (someone correct me if I'm wrong) but you should try the riffs for pretty much ALL of their songs.

What skill level are you? That would help us decide what riffs to give you, because riffs vary. Obviously. Really we need more description than "good shred/ black metal riff." Does that mean you want a shred lick or a riff that's hard?
Try Yngwie's "Deja Vu" intro riff, it works your minor thirds and phrasing over two strings.
Joe Satriani's "The Extremist" has a good bluesy intro riff and some great licks in the verses.
Racer X's "Scarified" will work on your 3 notes per string riffs and licks as well as arpeggio skills.
Technical difficulties, also by Racer X, is a big one in my area, lots of guitar players know it and it's great fun just to sit down and start playing and all of a sudden four other guitar players join in :P It works on your picking technique and your speed as well as your 3 notes per string ability.