Does foam rolling cause bruising? The answer is no. If after foam rolling you see bruises on your body, you need to regress yourself to a softer, less dense foam roller. So if you're using the black foam roller, I would definitely take it a step down and start using the white foam roller. If you are using a tennis ball, a softball, or any kind of harder, more dense ball, I would definitely take it down. Regress to the less density version of your foam roller.
Maybe you can also put one leg up. If you were lying down let's say on your IT band, you might have been applying too much pressure to that given area. So you could always kind of use your body weight and lessen the pressure on the roller by putting one leg up, by putting your arm down. You definitely don't want to experience any bruising when you're foam rolling, and if so, definitely adjust next time accordingly.