This is an easy issue to solve, I think. Immediately recognisable are two options -- either, A) the mud-blooded are sent to the respective nations of their non-White ancestor, or B) the mud-blooded are permitted to stay within the White nation of their birth and permitted only to marry other mud-bloods, essentially remaining second-class citizens.

But there is a third option. Why not simply establish a mud-blood nation, or even several? Why not use South America, as an example, to establish a mud-blood homeland? This way, all issues find their solution. Firstly, the mud-blood is not treated as a second-class citizen, because he is essentially among others of his breed; second, they are not restricted to a very small percentage of the population when choosing a mate, because mud-bloods are mud-bloods and they do not have any race to preserve; and finally, and perhaps most importantly to us, they are not in White nations -- of course we all know the advantages that presents, it doesn't need to be elaborated-upon.
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SIEG -- oder BOLSCHEWISMUS?

May all your Christmases be White.