It's a complicated discussion and you should be careful not to offend others (ofc it's not always easy). There are no black or white facial features, you could say someone has more western looking features or more African looking features in a general sort of way but those two area's do have a wide verity of features.
Vivienne is on the dark side and she has more African features (cheekbones, jaw line, lips) but a woman who didn't have those is not less black because of that and how dark or light a person is doesn't make them more or less black either. Black is more of a cultural identity than a classification.
There is some prejudice both outside and inside of black communities against darker women so it's nice if they are represented. Whether or not Vivienne is romanceable doesn't make a massive difference to that though (in my opinion). I guess you could say that it shows bioware see her as an attractive desirable person but I already know they wouldn't leave her out for being (too dark) so it matters not, to me.