And no. It's not racist having an opinion on what Rarity would look like as a human. As long as she is just a fictional character, then no harm is done. I was pretty suprised that you chose Rarity to be black, since she is white in the series, but that is just Creative, and your opinion. And i like the way you think. Again i'm sorry if any of this sounds racist.. And sorry for my bad english..
I don't see Rarity as a black person either. (Not racist) In the series she is white, and she likes to be very royal/lady like. Like in the old days. And in the old days, people were very pale. So if i saw rarity in a human form, i would think pale. (Not pale white, just palER) But that is just my opinion, i like your Rarity, and i respect and like the way you think about her, in the human form.