We have brown rugs with a wide black border. Sometimes our roomba red sees the border, and retreats as if there is a cliff. Other times the robot would get on the border and stop believing it was in space. It would also pass over the border and get trapped on the rug, resulting a very clean rug.
I think the speed of the approach and maybe even angle determines how the robot will respond too a black rug. Bottom line it is not supposed to work on black rugs.
Reds x 3, Dirt Dog, Disco (now a parts bot), Create, Scooba 350, and Security Dawg