In the Marvel comic "Thor" they portray the goddess Sif as a brunette which is contradictory to her mythical self who was titled Sif of the Golden Hair. Was there ever an explanation for this difference.
When Sif was a child, she possessed long golden hair. She often played with young Thor, Odin's son, and Loki, Thor's adopted brother. Sif and Thor fell in love while they were quite young. When she was kidnapped by Storm Giants and made a prisoner of Hela, Thor offered his own life to ensure her release. Hela, impressed by his nobility, released both of them. Later, jealous Loki cut off all of Sif's golden hair while she was asleep. This is the Marvel version of the classic tale of how Sif lost her golden hair.
This tale of Loki stealing Sif's hair is one found in several tales of legend. You can read the tale here: Sif's Golden Hair: How Loki made mischief in Asgard.
Guessing that Loki had something to do with Sif's loss of hair, Thor angrily demanded that Loki restore her golden locks. The scared Loki contacted two dwarfish craftsmen, Brokk and Eitri (who would later create 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. --Marvel Sif Wikia Entry