Wirts leg is sometimes black...hrm...
Try and search for this. RTB may have posted something about colors a looooooooooong time ago. Erm....search is up "there" somewhere...try and find it. IDK, search for "Black Item Mods" or something...
That Level 2 unique shield is either White or Black...and the Level 2 Sword (ohh, Rixot's Keen) is the other color. Thinking about it now, I think that it's Rixot's = Black, Shield = White. Should be cool...
Corpsemourne is a black armour (darker than IK) and looks awesome on a Druid. Ormus Robes is also black.
Either (both?) Guillame's Face or Howltusk is a black helm.
Tiamat's Rebuke is black and grey.
Can't think of any others off the top of my head.
Putting Perfect Skulls into a non Unique/Rare/Set item will turn it black. It's possible that the shade of black depends on the quality of skull so that a chipped is very dark grey while a perfect skull is black, but I'm not sure.
Steelclash is a black Kite/Dragon/Monarch style shield, if you want that look.
I think Guillaume's is just the normal grey colour of Winged helms, I don't think it has any colour modifier. I know this because I used a pally with Guardian Angel and Guill's, and Guill's was definately lighter in colour than GA.
I keep on waiting for my Valk to spawn with a sexy blacky spear, cause that'll mean either Cruel or of Butchery (or a similar mod), both of which mean she'll be doing upwards of 5k per hit. So yah, Ferocious/Cruel or the "of Butchery" mods both turn it black.