According to this Steam product listing lifted from Steam files, listings for OSX files for Call of Duty: Black Ops and its related DLCs are present.

The last game to be released natively for Mac OSX was Call of Duty 4: Modern Warfare. Between the OSX and Windows versions, there are essentially no differences. If Black Ops is released on OSX, it will be the first Call of Duty game to be released on OSX via Steam.