The main benefit of an interracial relationship is that the two people in the relationship get to be with the person that they love. This is really the only good reason to be in a relationship. Therefore, the benefit of an interracial relationship is really no different than the benefit of a relationship with a person of one's same race.
You can also argue that interracial relationships are beneficial to society as a whole. You can say that these relationships lead to more understanding between (in the case you are talking about) whites and blacks. More interracial understanding would reduce levels of racism and make our society more united.
However, the major benefit of such relationships is to the people who are in them, assuming that they truly love one another.