With IR, you need to assert that the black is still black, as lots of black stuff is "the new white" when using IR light.
I don't think that you will have that much luck using IR-LEDs and integrated receivers, as this combo will be quite wide eyed.
A simple red LASER pointer (modulated perhaps) as the source and a (wide eyed) photo transistor, OR, a wide beamed LED and a phototransistor with a narrow angle of detection (either ready made or with DIY optics) would be the way to go.
Using visible light will make it so much easier to test and align it as well - and it will adhere to the 70's hit: Black is black.
At 20cm distance, the black part is a maximum of 14cm wide (if rotated so a corner is pointing towards the detector) but still only 5cm in the vertical direction - this takes one of the devices to be a rather narrow beam (photo transistors with a 5° angle should do nicely).