I tend to find the usual tropes in interracial content to be rather cringey, and I'm not a big fan of the cuckolding route(of either race).
This might seem rather tame, but maybe explorers(can be white or black) come upon the civilization of the other race(opposite of your choice) and there initially was an attempt at diplomacy. The elites of each group have callous attitude toward the other, yet in the fringe frontier it's not uncommon to find consensual intermarriage/hookups among the commoners. Over time, there developed commoner villages of mixed people born from the unions of both white men/black women, as well as black men/white women. These mixed villages could be the "underdog" factor that represents the disenfranchised peasants that have little in common with the racist power hungry lords of each kingdom, preventing the masses from advancing.
Now fantasy interracial with human supremacy over non-humans is where I become a bit more racist(speciest?) lol. Fantasy setting as OP suggested, though humans and variety of different non-humans. Humanity has the social domination, officially sponsoring the policy of "civilizing" and "assimilating" the non-humans. This doesn't mean blatant rape or forceful cucking, but encouraging interracial unions(interspecies) to dilute the non-humans. The human holds superior status in the relationship regardless of their gender. Non-human men are encouraged to marry human women, clearly they should admire the beauty of civilized human women over their inferior non-human females. They'll serve as proper knights for human military, their mixed offspring will take their maternal human surnames and civilized culture. Human men are free to marry non-human women, or simply have illegitimate offspring with them. These non-human single mothers of mixed bastards might encourage their progeny to further assimilate into human society with help from their mixed features. The central theme is to dilute the non-humans through less violent means and the future mixed generations uphold human values.
