This is not James Bond. Steve Jobs was a real person. And Steve Jobs never had blond hair. Heck, Michael Fassbender doesn't even have naturally blonde hair. His hair is a dark brown. Why would the filmmakers go out of their way to make him look nothing like Steve Jobs when the movie is called Steve Jobs?
The makers didn't care about him looking like the real Steve Jobs:
According to the film’s director, Danny Boyle ("Trainspotting"), the decision to not have Fassbender look like Jobs was made very early on.
"We were very clear right from the get-go that this wasn't about a physical impersonation at all," Boyle told Business Insider. "It was about inhabiting, it's almost Shakespearean. He's historically accurate to a degree, but then he is our version of him, and there will be other versions."
Sorkin, who also spoke with Business Insider before the film premiered at the New York Film Festival over the weekend, said he instantly agreed with Boyle's instinct.
Sorkin said the way Fassbender looked in the new film "just isn't important."
According to Entertainment Weekly:
What did happen, however, was that the production team kept wigs and makeup on hand throughout the process of making Steve Jobs. If Fassbender and Boyle changed their minds and wanted to move closer to Jobs’ look, beyond the brown contacts the actor wore, they had the option.
According to some, Fassbender's natural hair color is blond:
The first full-length trailer for the Danny Boyle-directed Steve Jobs biopic is here and it's gripping. Michael Fassbender nails his performance as the charismatic Jobs, despite not looking like him. It appears he didn't even dye his blond hair for the role.
However, other sources describe it as light-brown or red.