Originally the dims were red and republicans were blue. Sometime in the late seventies they flipped the colors around. Back then, there were only the big three networks and they realized that the red comparison would not be good for their agenda.
I found some old videos on YouTube of 1980 election coverage - looks like Reagan’s states were blue and Carter’s red in that election. In 1984 and every election since then in the election coverage, Republicans have been colored red and Democrats blue.
http://www.youtube.com/watch?v=JTMpQqDP-nk - 1980 coverage video
http://www.youtube.com/watch?v=Ev_9YcM-HNI - 1984 coverage - video
This is where the massive amount of (mis)information on the internet starts to show.
Not long ago, it was possible to quickly find a chart showing red/blue maps for the last several decades of elections, with notes explaining that in this or that year, red = this party, and blue the other.
My memory was that it flipped every election to avoid any preferential argument being made by one side or the other.
Not only can I not find that set of maps, the sheer number of references to the post 2000 “norm” has buried it. The closest I can find is the following link, which actually defines it a bit differently, though perhaps more accurately than I recall:
http://www.washingtonmonthly.com/archives/individual/2004_11/005157.php