It is easy to understand why family bloodlines carrying known familial witchblood tend to associate with and marry into other family bloodlines also carrying known familial witchblood. In medieval Christian Europe, deeper than simple ethnic ties, it was a matter of survival and mutual protection from church authorities and witch-hunters to stay closely linked together.
In modern times, even though the knowledge of witchblood is lost to many, genetic evidence has shown that some maternal and paternal genetic haplogroups have traveled together for eons of time. For example, I2 and U5 genetic haplogroups (my own paternal and maternal lineages) have been found together in families in ancient human remains for at least as long as 50,000 years. I2 males and U5 females have gotten together in close familial kinship ties for a long, long time, particularly in Europe. Ethnicity doesn't explain it all, as other genetic lineages, notably R1 and H, are highly common in nearly all European ethnicities (and have witch bloodlines particular to these haplogroups as well).
It is conceivable that supra-conscious spiritual kinship (through shared witch bloodline connections down through the ages) have influenced continued associations between particular families, no matter from which ancestral lineage haplogroup they descend. This is especially true when some families may have continued to value witch bloodlines even during the reign of political Christianity in Europe.
Preserving witch bloodlines (against extermination and extinction in Christian Europe), despite the loss of overt recognition, may have been the impetus driving the formation of many European secret societies. For example, my Faulkner lineage and many of my maternal lineage males are known to have been Freemasons (members of ceremonial secret societies) for centuries, documented since at least the mid 1600's in America (and before that in Europe).
Among European Americans, known witch bloodline surnames associated with the surnames found among my own maternal and paternal ancestral lineages include:
Salem (1692) Witch Bloodline Names - Faulkner and Green (Greenwald, Grunwald), maternal lineage surnames; my own Faulkner family lineage came to America from Wales on the ship Agreement in 1665, decades before the infamous Salem witch trials of 1692.
Scottish-Welsh Witch Bloodline Name - Tailyour (Taylor), paternal lineage surname.
You can find other witch bloodline surnames here - Ancestral Witchcraft: Witch Bloodline Names from Salem, Scotland, Germany, Italy, and Scandinavia.
Many today believe that the accused witches of Salem were not real witches. I disagree. Of course, the families of those accused of Witchcraft in Salem in 1692 would not claim to be witches or of a witch bloodline. That would be suicide, and lethal for the whole family. But, the fact is, the family surnames on the linked list belong to the people accused (and condemned, if they did not escape or die in jail before conviction) as witches in Salem in 1692.
Clearly, to be accused, these family bloodlines did not fit into the mold of the average person living in Salem in 1692 - they were different. Misfits. Outsiders. Women with a power uncharacteristic of Christian tradition. Witches.
Many today also believe that Tituba, a slave from Barbados, was the only true witch among those accused of Witchcraft in Salem in 1692. I also disagree with this belief. My understanding is that Voodoo is not Witchcraft.
Voodoo is a combination of African native spirituality, Catholicism and Native American spirituality. In light of this understanding, Tituba was not a witch (a practitioner of Witchcraft), but rather, was a practitioner of Early African American Voodoo (which was probably closer in spirit to its Native African roots than it is in modern times when it has become a more blended tradition).
In moderns times, some have blended together Witchcraft and Voodoo into their own syncretic systems of magical practice. Perhaps the friendships of the accused Salem witches with Tituba was an Early American attempt at syncretic and mutually supportive magical practice as well.