Wikipedia

In American politics, a blue shift, also called a red mirage, is an observed phenomenon under which counts of in-person votes are more likely than overall vote counts to be for the Republican Party (whose party color is red), while provisional votes or absentee ballots, which are often counted later, are more likely than overall vote counts to be for the Democratic Party (whose color is blue). This means that election day results can initially indicate a Republican is ahead, but adding provisional ballots and absentee ballots into the count can eventually show a Democratic victory.