I think it means the Sephardi must have Ashkenazi blood, because dark skin is usually dominant. My light-skinned nephew from a mixed marriage has an Ashkenazi great-grandfather on the Sephardi side, so I think that’s how that happened.
My dark-skinned nephew is Indian on his father side so I don’t think there is any Ashkenazi blood there. He is very dark, but I don’t think my sister likes it when I call him an Indian and sing “one little, two little, three little Indians” to him.