Mariah is white-passing, but biracial and self identifies as black. Not being able to immediately perceive her race doesn't change what she is and how she self identifies.
But anyway, most biracial black people that have a more obvious racially mixed look (like Alicia Keys' phenotype) are pretty much just considered black and most self identify as black as well. The history and dynamics of "blackness" in the US is unique. Black isn't sliced and diced up by who looks like what -- if you were known to have black ancestry back in the day, you would have been in the colored lines during Jim Crow era and a slave all the same 100 years prior. And most white passing biracial people didn't take the risk of trying to live as "white" considering the dangers and the fact that they'd have to abandon their families.
It is what it is. Today it's not quite as cut and dry as "one drop" used to be and people accept those that self identify as biracial/multiracial as well, but a lot of people feel it's easier and simpler to just say black and keep it pushing.