Ancestry in the Wizarding world.

Not a big deal, really. The Black Family was one of the most powerful, wealthy and influential families in the Wizarding World, especially among those with Supremist sympathies; it's understandable why other Pure-Blood Supremists would want to marry into a family such as that.

द्वारा the way...Septimus Weasley and Cedrella Black are the parents of Arthur Weasley, and if आप look आप also see Neville's folks. If आप look to the left, आप see Lucretia Black married Ignatius Prewett, who was a close relation to Molly Prewett Weasley (Percy's middle name is Ignatius).

And it's not just a desire to keep the Pure-Blood bloodline "pure", it's also a desire to keep the Pure-Blood bloodline British. Pure-Blood Wizarding families with Supremist beliefs would honestly rather marry within the family than marry a "foreign" Pure-Blood (this sentiment was evident even in the Weasley family who, though they are not Supremists, for the most part did not want Bill marrying Fleur; not because she was part Veela, but because she was French).


Aaaah...yeah...I have spent waaaaay too much time in Harry Potter's world. :)) :p