Sif used to have golden blonde hair. One night Loki played a trick on Sif and cut off all her hair in order to ruin her and Thor's realtionship. Thor forced Loki to restore her hair, Loki went to two dwarfish craftsmen, Brokk and Eitri (who created Thor's hammer Mjolnir) and asked them to create new hair for Sif. Loki was either unable, or unwilling, to pay the dwarfs enough gold to create golden hair.
The dwarfs decided that "for the price of naught" they would make "strands of naught." The dwarfs created magical strands of hair from the blackness of the night. The strands of darkness took root the moment they were placed on Sif's head, and then began to grow out. Thor found that Sif's new black tresses made her even more attractive, disrupting Loki's plan to ruin their relationship.
