"Filth! Scum! By-products of dirt and vileness! Half-breeds, mutants, freaks, begone from this place! How dare you befoul the house of my fathers -"
-- Walburga Black screaming from her portrait (OP4)
Walburga Black was the mother of Sirius and Regulus Black and wife of Orion Black. Walburga was an enthusiastic supporter of pure-blood superiority. Her prejudice against those she considered to be inferior, especially “mudbloods” and “blood traitors”, survived her death in the form of a vitriolic portrait hanging in number 12, Grimmauld Place. During her life, she was fond of zapping family members off of the Black Family tapestry, including her son Sirius who left home when he inherited money from his Uncle Alphard, as well as her niece, Andromeda, who married a Muggleborn named Ted Tonks.
She was either very old (especially when you consider that her children were so much younger) or unnaturally aged, judging by her appearance in the portrait in Grimmauld Place.
Daughter of Pollux and Irma (Crabbe) Black.
Two younger brothers: Alphard (disowned) and Cygnus (1938-1992).
Married her second cousin Orion Black, so both she and her husband were descendants of Phineas Nigellus Black (also confirmed by BFT). This explains why she refers to the House of Black as the "house of my fathers" (OP4).
Walburga is a saint in the Roman Catholic tradition. She lived in the 700s AD. Her feast day is May 1, and the evening before is known as Walpurgis Night. Legend has is that on this night, a large meeting of witches takes place on Brocken, the highest peak in the Harz mountains.