Black isn't a race
if you are using it to mean the race, then obviously if the person is mixed with Black (a lot, not like 10%) then they are
but if you're talking skin color and they look real light then they obviously aren't "black"
plus what people consider "half black" "half white" etc isn't ever truly "half"
i know kids with both parents that are black but they turn out lightskin or light brown
would you consider them "black", race wise yeah but color wise no
and sometimes kids with a black and white parent turn out to have white skin, but in reality have a black parent
would you consider them black? not color wise ofcourse, but race wise you have to
so it's all about context
if you mean the race or the color