What's actually happening: According to Richard Whitney, VP of product at Particle, the soap dispenser uses near-infrared technology, which sends out invisible light from an infrared LED bulb for hands to reflect the light back to a sensor. The reason the soap doesn't just foam out all day is because the hand acts to, more or less, bounce back the light and close the circuit. "If the reflective object actually absorbs that light instead, then the sensor will never trigger because not enough light gets to it," Whitney told Mic.
Whitney presented two extremes. If someone were to put a mirror up to the sensor, the light would reflect easily, triggering the sensor with no problem. But a material like vantablack, which only reflects 0.035% of all light pointed at it, would be kryptonite to an IR sensor.
"In order to compensate for variations in skin color," Whitney said, "The gain, [or] sensor equivalent to ISO and exposure in cameras, would have to be increased."
Trashy website, but it gives an answer.