Scientific analysis has proven that you can't really reliably identify a person's ethnicity based on their appearance. She could have black ancestry, but it's probably not a whole lot more than 15% or so. Sometimes you can see such traces, and in other cases, you can't.
Actually, she's a lil' too cute to be with him...how the hell!!?!!! This is damn near Janet Jackson/Jermaine Dupri shit....but probably more Mariah/Nick Cannon.