I'm caucasian, fair, blonde and blue-eyed. My son is Guatemalan, with gorgeous dark skin and eyes, and an incredible head of dark hair. When we're alone together, I most often get asked if he's mine,to which I simply reply: "yes!" I have had a few folks ask if my husband is Filipino/Hispanic/Mexican, and I usually answer no and leave it at that.
If people press, and they have been kind and interested, I generally am willing to tell them that we adopted my son. Although I know a lot of people believe that it's his story, I am incredibly proud of his story, how unique it makes him and people are almost invariably supportive and interested in it.
I think it's fine for you to just say, "curly hair must come from my husband's side." or "we can't imagine where it came from either!" (Actually, that was a common comment my folks used to make about my sister and her curly hair. She is their bio child, but the ONLY one in generations with ringlet curls.)
Kelley