The title sounds like the OP wants to know when white women should breed with "superior mixed black babies", which, if I can recall from my 5th grade sex ed, is an impossibility since babies are incapable physically of producing sperm.
And, if the OP takes the position that blacks are superior, then how would a "mixed black baby" be "superior" since that human being isn't black? Wouldn't it only be half superior, say superior for 6 months of the year and then inferior the other 6 months? Or if superior is at one end of the scale and inferior is at the other, wouldn't mixed be "equal", or right in the middle, like doing a ph test and the result is a 7, neither alkaline or base?