To answer the question, it was the media. Especially the electoral map for the 2000 election.
1992.
I know this is correct because I had a screaming fit about it when I saw them doing it in 1992 to disguise the fact that Bill the human sh*tbag was a Communist who toured Russia in 1968, protested American involvement in Vietnam, and who stayed with well known communists when he toured countries behind the Iron Curtain around the same time.
Bill Clinton was getting so many linkages to communist organizations and people, the media colored him "blue" as a smokescreen.
I used to have examples of election maps in various newsrooms, but I can't find those anymore.
But the change occurred in 1992. Reagan and Bush HW were "blue."
I know, I used to distribute their yard signs.