I have two Roomba Discoveries and niether one of them would roll over BLACK color lines in any of my area rugs. They would stop at the edge of the BLACK and change direction as if they detected a step or ledge. I contacted Irobot customer support. They couldnt help so I experimented and found a solution.
Step 1: Locate Roombas Step Sensors
Note: this fix is meant to be used only when you run the roomba in an area where it will NOT approach ledges or steps since the roomba will run right off the step and fall. So just line up a pair of shoes end to end across the top of the step or ledge so the Roomba will bump the shoes and change directions.
To fool Roombas ledge / step sensors so it will run over Black colors in rugs etc. we need to locate the sensors affected in this tip So first, turn your roomba discovery over ( I have the "Discovery" model, this may work on other models, just try and see) and locate the four sensors along the bottom edge of the front bumper. if you look down inside the openings you see along the bottom edge, the sensors look like little L.E.D. lights poking out a little bit.
Step 2: The "MASKED" Sensor :-)
Put a piece of masking tape over each of the four sensors located along the bottom edge of the front bumper. The senors will not be able to see anything but the tape and it will run over top of black color patterns in rugs.
Step 3: Sens...less Vacuuming :-)
Now the Roomba will roll over any colored rug....remember to block all steps and ledges while the sensors are covered. When finished simply pull off the tape and the Roomba will regaine its senses :-)