DirectX 11 is only available on games that support it. Call of Duty Black Ops does not support DirectX11 and will only render in DirectX 9. Right now there are only maybe 20 or so games out there that support DirectX 11, and none of the Call of Duty games are on that list. The ones I can recall off the top of my head that support DX11 are:
Metro 2033
Tom Clancy's HAWX 2
Lost Planet 2
Aliens vs. Predator
Crysis 2 (with DX11 patch)
Total War: Shogun 2 (with a DX11 patch)
Dragon Age 2
World of Warcraft
Dirt 2
Most games do not support DX11, and that trend is likely to continue for the next while, it takes time for developers to implement new technologies, and the consoles can't support any DX11 features, so there is little motivation from some developers to implement DX11 on their PC ports, especially if they view the PC version of the game as a quick and dirty way to make a little bit of extra cash.