This is a tough one. I have similar, although it's not racial.
I get comments about my children being "dark" like their dad. One friend looked in the crib and said "I guess there wont be any blond babies!" and last weekend, a friend of my dh's said "None of them are blond like you? You didn't get any blonds?"
It always bugs me because it makes it sound like the fact my kids are not light like I am is somehow a loss. Often, and this is not a comeback everyone can use but my whole family has brown eyes and black hair (well obviously there were a few blue eyes floating around but we have never figured out the hair...). I was a bit of a joke growing up (the mailman, etc.) and felt very self-conscience that I didn't look like the other members of my family.
"No, I guess this blond business starts and ends with me!" or sometimes I say "Well, at least they look like the rest of their relatives-on my side too!" Then, it's obvious that any "blond" comments might mean insulting my whole clan lol!
I wouldn't worry about it too much. Call her out if it's overtly "racist" or insensitive. For example, if she says something about your child will be good in Math automatically because of his background. Use some sort of line about not reinforcing stereotypes or pegging people. You can keep it light and change the subject. If your child picks up on any of this, just tell them that older people have some weird notions, old fashioned, out-of-date ideas, etc.
If she gets out of hand, have your dh talk to her. It's his mother and his responsibility. This can be a revenge tactic for having two children marry people of different backgrounds. Don't play into it. Many grandparents rile up their dil's and sil's, only to be nice as can be to the grandchildren.
It also might be the adjustment to letting go of her fantasy of having white, blond, blue eyed grandchildren. She may have pictures of her friends' white, blond, blue eyed grandchildren shoved in her face on a regular basis and this might make her feel that hers' wont measure up.
Let her get over this and give her time and space to get a grip on reality. In time, she'll establish a relationship with her grandchildren and be able to appreciate their individual merits. The whole race thing wont matter, or matter as much. Right now, they're small or not born yet and she's obviously dealing with some issues. When her grandchildren are real human beings, showing off artwork, good grades and cartwheels, you may see a real change in her attitude.
We all know that it's easy to love children who don't have any of your physical traits. Adopted people love their children just as much as biological parents do and having a child look like you doesn't increase the love between a parent and child. Expectations can be tough to let go but they can be reshaped. That just has to sink into her heart!