All I can say is stick to your guns. And as Dianne said, what is your Mom going to do, ground you?
Be glad you aren't confined to "obeying" your mother. It's your life.
I would confront my mother and very firmly forbid her to ever say anything negative to my daughter or anyone else, about any person I choose to go out with. If she doesn't comply, then I would keep my distance from her.
She'll have to keep her thoughts to herself if she doesn't want to lose you. It might be tough, but be firm, and don't allow her to draw you into a discussion about it.
Our son married a black girl from Texas. We all love her dearly. She has brought a light into our lives like no one else ever could.
Hannelore