According to Arthur Hove, a historian of UW-Madison, the cardinal color (sometimes called "Wisconsin Red") and the white accent color were "generally accepted as the school colors" in the 1880s.
The addition of black, which started appearing sometime within the past 10 years, has been used in an effort to add contrast to the cardinal and white. But it seems that black is, and shall remain, an accent color.
You may also have noticed that UW-Madison recently changed its logo. On this topic, Chancellor John Wiley has said, "Consistent use of the new logos is an important first step in a campuswide effort to present a clear, unified UW-Madison identity."