I would argue that there are not any such passages. The eNotes link below gives a number of verses that it claims are warnings against interracial relationships. However, I think that those are rather tangentially related to the subject.
I would suggest that you look at the other link below in which someone from Bob Jones University in South Carolina (which used to ban interracial relationships). There, the spokesman specifically states that there is no Bible verse which directly teaches against interracial relationships. Instead, the article says, the ban on interracial dating is based on the principle that God does not want all the people of the world to come together. This was why God did not allow the Tower of Babel to be built.
So, there is no Bible verse that directly condemns interracial relationships.