A lacklustre Chelsea were meandering towards a 1-1 draw at Ewood Park after Nicolas Anelka had cancelled out Benjani Mwaruwari's opener.
But six minutes from time Yuri Zhirkov's cross was headed home by the Serbian defender Ivanovic. There was disappointment for Manchester City, who were beaten 2-1 at Wolverhampton Wanderers, their second successive defeat.
Chelsea lead the table with 25 points from 10 games, five points ahead of Arsenal and eight clear of City and Manchester United, who were hosting Tottenham Hotspur later on Saturday.
At Ewood Park, Blackburn, who are tipped to be taken over by Indian group Venky’s, started the brighter of the two sides, with the Chelsea players looking jaded.
Zimbabwean Benjani Mawaruwari finding space and time. Chelsea could hardly complain when Blackburn took the lead midway through the first half, with Benjani being the scorer. John Terry and Alex appeared to have a routine cross from El Hadji Diouf covered until Benjani beat them to it to register his first goal in almost two years.
Anelka equalised for Chelsea shortly after, connecting a Florent Malouda cross. Blackburn again looked the more likely to find the winner before Ivanovic scored against the run of play with six minutes to go.
At the Emirates, West Ham goalkeeper Robert Green put up a heroic performance to deny the Gunners a goal until two minutes from time, when Song headed in from six yards out after neat work from left back Gael Clichy. Arsenal manager Arsene Wenger made nine changes from the team that so effortlessly defeated Newcastle United in the Carling Cup and the team looked no less destructive with the Arsenal midfielder immediately seizing control and creating several chances within the first ten minutes.
But as has been the problem with Arsenal sides of recent years, they were unable to make most of the chances. Theo Walcott and Nicklas Bendtner were brought in midway through the second half, with Walcott immediately making an impact. With Green in great form, the match seemed headed for a draw before Song’s timely intervention gave the Gunners maximum points.