Utter and complete nonsense.
During most of history, most people could have cared less what color a man’s skin was, although it would startle them if it were unusual for the area. What was important was a man’s wealth, social position, craft skills, and so forth.
People who married within their tribe also could care less what a man’s skin color was. If he were adopted into the tribe, he was a tribe member. If he came from just over the hill and looked exactly like your tribemembers but was unrelated, he was a stranger and a foreigner. And as has been pointed out, faithful Jews did a lot of marrying of converts. This is why there are Jews in every nation, of every possible skin color and appearance.
From the beginning, Christians came from every nation, and they intermarried a ton. They adopted babies left in the sewer or on the dunghill, raised them as their own, and married them to their relatives. Christians took the children God gave them; they didn’t have any compunctions about race. What was worrying was a marriage that mixed religions, not ethnicities.
Which isn’t surprising, because the people of the Roman Empire weren’t particularly concerned about race, either. They might tease you if you married some blond German, but that’s about it. Anybody could be a slave, anybody could be free, and anybody could be some nouveau riche guy trying to marry your daughter. The only time anybody got upset was when it looked like some Bulgar would get to be Emperor; and that was about enemy countries, not race. The legend of Romulus proudly proclaimed that all the great families of the city of Rome, like those of Australia, were descended from criminals and fugitives from all over the Mediterranean.
The Roman jurist Modestinus, who was not even Christian, said of marriage under Roman law, “Marriages are the union of male and female, a sharing of life and the communication of divine and human rights.” There is nothing here about race or tribe.
Pope Pius XI’s syllabus against racism lists as one of the false and heretical propositions which must everywhere be refuted, “We must by all means, preserve and cultivate strong race and purity of blood.” Using any kind of eugenic reasoning to prevent the marriage of a man and a woman who are capable of matrimony under natural law is “pernicious”, according to his encyclical “Casti Connubii”, because “men are begotten not for the earth and for time, but for Heaven and eternity.”
With writing help from the man who would succeed him as Pius XII, Pius XI also sent forth the great letter “Mit Brennender Sorge”, which further denounced blood purity ideas: “The peak of the revelation as reached in the Gospel of Christ is final and permanent. It knows no retouches by human hand; it admits no substitutes or arbitrary alternatives such as certain leaders pretend to draw from the so-called myth of race and blood.”
I will mention finally that it’s not so many years ago when a Catholic priest, like St. Valentine himself, was murdered for daring to marry two people whose parents didn’t like the match – because she was an Alabama white Catholic girl and he was Hispanic. Christianity is the religion of love, and the Church believes in the unitive power of marriage.
The Church has traditionally discouraged marriage within the seventh degree of consanguinity either by marriage or relations on your own side (including baptismal sponsorship relationships and adoptive relationships) and required a special dispensation for first cousins (the fourth degree of kinship) to marry at all. So there’s a definite discouragement of marrying one’s kin.