I don't believe that Hollywood created scripts with black or women presidents in an attempt to prepare us for the day when such an occurance developes in real life however I do believe that Hollywood has become much more responsible with showing minorities in roles where they are quite competent with their position.
In the hit series, 24, they had two black presidents who were very believeable and very competent. In the short lived series, Commander In Chief, Geena Davis played a very competent president. These shows may have unwittingly prepared some of us for minority presidents but I do not believe the major intention was to make us cringe or rejoice - I think that it intention was to say that anybody, regardless of race or gender, can be competent in their jobs - even if their job is the president of the Unites States of America.
Colin Powell was a very competent General. Thurgood Marshall was a very competent US Supreme Court Justice. Denzel Washington and Morgan Freeman are very competent actors. Why couldn't a black man be a competent President?
Dr. Condoleezza Rice became the 66th Secretary of State on January 26, 2005. Senator Hillary Clinton has been very successful in law and in the Senate. Both are competent women. Meryl Streep and Glenn Close are both accomplished and competent actresses. Why can't a women be a competent president.
My point is that by being responsible and giving these minorities roles which reflect real life competencies, they have also prepared us for minority presidents without having that as their main objective.
It is called progress.
Thank you for a great question.