I think there is many underlying issues there, for one she doesn't like the fact that you have taken her son away from her and like you said the fact that she is now alone, but she has to realize that her son has a family that needs him now and that her job as a parent is pretty much done and she has to let him go live his life, that is why we as parents teach our children the tools in which to do that, it's not your fault that you were raised by white parents and really shouldn't make a difference and I would like to know how a real black woman acts? All woman act the same they raise their children and protect them and just because that white family was not your biological family they gave you the tools to be a mother and a wife it shouldn't have anything to do with color but unfortunately it usually does in situations like this. It isn't just white people who can be prejudice black people can be also and she seems to be living proof of that, it is possible to get along with your mother-in-law, and it can also be great for you, your family and your marriage, The troubled relationship between men and women and their mothers-in-law is cited as one of the number one reasons for problems within a marriage, even more so between women and their husband's mother. Getting along with your mothers-in-law is even more important when there are children involved.
Regardless of your personal feelings toward her, never criticize or speak badly about your mother-in-law in front of your son or your husband, or any other members of the family for that matter. It will only get back to her and make things worse for you.
Maybe if you include her more she won't feel left out and alone, invite her to dinner more often it doesn't have to be every week but if she sees you making an effort she may also make an effort but I really don't think it has anything to do with you I really think her resentment is towards her son and she is just taking it out on you because she feels her son abandoned her and because he married you she thinks it's because of you partly, maybe if your husband went to his mother's more often to make sure she is okay she will in turn let up on you. It's a simple matter of respect: She doesn't feel she is getting the respect she deserves from her son and is blaming you for that. Try to see things from her point of view, take the initiative to be pleasant and approachable. Even if your efforts go unnoticed, you'll at least know you're doing the right thing, and that in itself is rewarding and enough to be commended and your husband will see that you are making an effort to let bygones be bygones and maybe he will talk to his mother about the way she is treating you if he sees that she still even after you make an effort still treats you badly he will see it isn't you. Try asking her out to lunch to talk and then ask her what you can do to make the relationship better she will appreciate that you at least care, she has already shown that she can be nice to you and this can be a constant things if you both try.
Sometimes just stopping and reminding yourself that your mother in law is a person too, a person with real feelings who has already lived a whole life in another generation before you came into the picture may be enough to put things in perspective. You should not allow the problems with mothers-in-law cause problems in your marriage honestly isn't beneficial to anyone involved, either now, or in the future. You may have to end up just loving his mother from afar, your families simply need someone to step up and take the high road, be the "better person" and do whatever it takes to make an honest effort toward improving the relationships. Maybe you could even plan a weekend getaway with your mother-in-law if feasible I think if you both had some alone time together it would make a huge difference maybe plan and Spa day for just the two of you. Call her sometimes out of the blue and ask her if there is anything she needs or if she needs your help for anything. She just wants to feel wanted and needs some attention to know that she still means something in her son's life.