It is a result of the systems being trained based on white adults
It's both. The system is racist because of how it was trained and because its developers were not black, therefore "it worked for them" during development. And because black people are harder for cameras to see, especially in low light environments.
Even with clothes on, the dark skin, in a dark environment, "breaks" the "this is human" pattern that the ai expects to see, since the ai can see only the clothes. It is like camouflage. Can the ai "see" a pair of pants? Maybe, eventually but it still reduces the certainty, since the ai sees fewer "signs".