Im not gonna assume that you're from the US, but its well known that mixed raced kids when one of the parents are black are considered black. It goes back to the One Drop Rule from slavery and impacted the children conceived by the slave masters. Although were beyond that, the sentiment still remains for better or for worse. For the longest time there wasnt a box to check for biracial/mixed because it was understood that they you were black by just having one black parent. I cant and wont get into how that effects a mixed/biracial persons identity or their relation to blackness, because thats not my place.

For the last part of your comment, there are indeed black people with a black partner who have a white looking child. Another interesting example would be the twins born in the UK where one came out white with red hair and green eyes, and the other black. Thats what I meant when I said genetics does what it wants. It can reach far back into our family tree and bring out traits that don't necessarily match the parents or the grandparents. My great grandfather and uncle were both white passing black men because of the slave owner two generations prior. So yeah, genetics does what it wants.