In the movie you see Victor is constantly changing, evolving, as the years go on. His hands/fingers getting more feline, his talons (er, claws) getting longer/sharper/staying out. At the same time, Creed got more aggressive, thought out his actions less. He was losing his humanity as he became more of a beast. By the time of X1, he has super long blond hair and is as mindless as a bag of bricks.
It's part of Sabretooth's ongoing mutation. Just like Wolverine. Some Mutants are born with their mutation (Changelings like Multiple Man), most get a single mutation at puberty, and some experience secondary mutations (Gambit, Beast had two, maybe three, if you count the experimental drug he took). But Sabretooth and Wolverine, whose healing factor has them physically age very little compared to what they should, continue to change. Except that Wolverine's on-going mutation is retarded by the adamantium laced into his body. His healing factor cannot get rid of the adamantium, and it has to fight against metal poisoning.
When Magneto ripped the adamantium out of wolverine in the comics, he starts to turn into a beastial form, much like Beast does. Hunched over, more muscles, flatter face, hairier, sounds dumber and harder to speak, more instinctual than logical, just like Sabretooth is.
But no, Comic Sabretooth, even in the Origins comic, has always been blond. The movie producers cared more about portraying the character properly than matching every single physical feature.
