The Blacks seem to me to be a family surrounded by tragedy. They live in a rich, luxurious mansion and wear the latest fashions made of the finest materials, and dine on gourmet food ordered in from Paris and Italy. They are treated like royalty, princes and princesses every one of them. But they're on a verge of bringing about their own destruction, their own slow and painful deterioration of the noble and most ancient house of Black. Because their greatest flaw, their one self-destructive feature, is their love. They love too much, and they love all the wrong people. The Blacks are living in a constant Romeo & Juliet tragedy, loving with their whole hearts, only to have it broken by whatever means. They're eaten up from the inside by their own love. They love a person they could never be with, and when that love proves a disaster, it breaks their heart and pulls them towards their own death and destruction. It's a curse that just surrounds that whole family and every member in it, and it reaches out to anybody involved with them.
(Focuses mainly on Bellatrix Lestrange, and what sort of person she is with what sort of loves and loyalties, because I really can't make myself believe that she's nothing more than an evil sadistic psychopath. She seems to be under that Romeo & Juliet curse stronger than any other family member, really.)
The noble and most ancient house of Black (and the Romeo & Juliet curse that they're all suffering from).