Since you are Christians, I would first ask her if she thinks rainbows are pretty. If she says yes (and what girl wouldn't?), then tell her that God made people like He made the rainbows...everyone is a different color, and all are very beautiful in their own way. Show her a picture of a rainbow and point out each color, and remind her that each color works perfectly with the other colors. Well, that is how we as people should be, too.
Even white people are all different shades of white. I am much paler than any of my sisters, and we are all white, but some of them have somewhat darker skin tones than do I. Black isn't the only color that has shades, and whichever shade she is, she is the shade God made her, so God must have thought she was something special to give her her very own color.
My own two children are bi-racial, and they are beautiful. They look like they have a tan all year round. I would love to be able to look like that. I am a natural redhead, and extremely pale skinned, so I only burn, never tan. I tell my kids I am jealous of them!
I always make sure that, if I buy my daughter a white doll, then the next doll I buy is black. Both kids know that they are bi-racial, and they do attend a mostly black school. There are some other mixed race kids there and a few white children. I feel sometimes that I wish they could be around my own family more, since they hardly ever get to see them, but my family lives 2000 miles away, so my kids are not that exposed to white people, but I do make sure that they know that they are the best of both races, and to understand that, although they look more white than black, that they ARE half black, and half white, and they need to know their heritage.
You also should make sure to buy some books and toys that are aimed towards black children, too. Don't just buy books with white kids on the cover, or dolls that are all white, or whatever. They need to have both things to help them understand where they are coming from.
I believe that mixing the races up will eventually, and it may take hundreds more years, but eventually bring about an end to all the racial b.s. that goes around. I wish we could all just think about the rainbow and remember that it really CAN be that simple that a child can learn it...so why can't adults?
Good luck!