I tried a lot of black materials including some auto stealth paint without success. It seems certain Ikea rugs and some hard floor materials fool the sensors. There was some "keep off" black material sold to fool Roomba sensors with mixed reviews; check Amazon
.  apparently now out of business -- perhaps because not reliable in the end.
For mechanical barriers consider right angle wood moldings and right angle transparent plastic wall corner guards in hardware (the plastic can be cut with scissors; get larger 1 1/8 in. size), to engage the bumper. A bit of velcro on the floor might be used to secure temporarily, all depending on the case. The plastic is good for making a wiring channel against walls for cords. Also 3M Command adhesive picture hanging strips like velcro but won't damage flooring with the glue.
 cover plastic with tape if wanted opaque.
The thing is materials black in the visible spectrum can still reflect in IR, besides which IR Proximity sensors are pretty bright IR emitters and the floor very close for brightest reflection, unlike weak lidar beams.