Help my fiance is blaming me for his mother dis likes for me, why?

We live in a diverse state, So his mother is used to my fiance being w/ biracial women or white women, basically women w/ long fine hair w/ some type of imediate caucasion family members. Im African american, i can p for biracial w/ my light skin tone and exotic look but the hair is all the way black. His mother and my fiance are also black ( & btw im not racist/prejudice or anything of that nature just trying to make a point)......when i came into the picture, he told me he never dated a black girl before . When his mom met me she imediatley was taken back by me,I think she was jealous to see a black strong women w/ so much spunk, drive, beauty and intelegence, she treated me and my son very poorly and rude. She has continuously made comments to me & my son in front of my fiance, and you know what, he said nothing! but i never let that get to me, i always put her in her place in a nice decent way that she would understand that i am not an easy target, anyway it's been 3yrs and it's simmered down a bit Ive always respected her when we are at family functions being a lady in all. but when im there at her house she normally caters to my fiance's daughter and my fiance and treat me and my son like we are invisible, it's very annoying but if im going to marry my man i just gotta deal w/ her ingnorance, unfortunaltey for my son. Anyhow she called me yesturday, btw she NEVER calls me unless she cant get ahold of my fiance, so imediatley im thinking okay ill just tell her he's not home, well she didnt ask for him, she actually asked how i was doing and how the kids were doing!! I was a little shocked but i didnt turn her away i spoke w/ politeness and was very attentive. however thats all she said, she didnt go any further. i expected that maybe she wanted to talk to me but then became afraid, i dont know, what should i do? i really want to be at peace w/ her and not have this costume on everytime i have to see her, i want to laugh and shoot the breeze like normal adult women but she is so sensitve w/ anything i say that she just ends up blasting out making smart remarks!! and then of course i gotta let her know how grown i am as well, anyway I told my fiance what had happened and that i would like to get along w/ his mom it's just that i know she isnt feeling me and deep down inside i know it's because im a black strong inteligent hard working women and she may not be used to that w/ her son. he told me that its both of us, and he has nothing to say about it.Okay, Although I dont think he should take sides, i do think that he should support me as a future wife to be and put out some suggestions, since it IS his mother, he would know best the kinds of things she likes/dislikes her personality etc, but for him to say that he will not discuss it kind of puts me in a diffucult place, because how do i know he doesnt discuss our personals to her, or feel like it's all me! what should a lady do?? Help Please!!