Blue states

Since the 2000 United States presidential election, red states and blue states have referred to states of the United States whose residents predominantly vote for either the Republican Party (red) or Democratic Party (blue) presidential candidates. Since then, the use of the term has been expanded to differentiate between states being perceived as liberal and those perceived as conservative.

All states contain both liberal and conservative voters (i.e., are "purple") and only appear red/blue on the electoral map because of the winner-take-all system used by most states in the Electoral College. However, the perception of some states as "red" and some as "blue" was reinforced by a degree of partisan stability from election to election—from the 2000 election to the 2004 election, only three states changed "color", and as of 2016 fully 38 out of 50 states have voted for the same party in every presidential election since the red/blue terminology was popularized in 2000.

The choice of colors reverses a long-standing convention of political colors whereby red symbols (such as the Red Flag or Red Star) are associated with left-wing politics, and right-wing movements often choose blue as a contrasting color. Indeed, until the 1980s, Republicans were often represented by blue and Democrats by red. The current terminology of "red states" and "blue states" came into use in the United States presidential election of 2000 on an episode of the Today show on October 30, 2000. According to The Washington Post, the terms were coined by journalist Tim Russert, during his televised coverage of the 2000 presidential election. That was not the first election during which the news media used colored maps to depict voter preferences in the various states, but it was the first time a standard color scheme took hold; the colors were often reversed or different colors used before the 2000 election.

...
Wikipedia