Chelsea suffered a stumble in the title race that could yet prove fatal to their chances of taking the Barclays Premier League crown to round off a week of bitter disappointment for Carlo Ancelotti's side.
A match that seemed there for the taking after Didier Drogba's early goal somehow drifted out of Chelsea's grasp.
El Hadji Diouf headed a superb equaliser as Blackburn put in such a powerful second-half performance that keeper Jason Brown was barely tested.
For Chelsea, whose record in their past nine matches is now four wins, four defeats and one draw, this was hardly a display of title-winning credentials and they are now four points behind Manchester United and two behind Arsenal, though with a game in hand.
This was underlined by the fact that the clear man of the match was none other than Rovers' 18-year-old central defender Phil Jones, making his league debut and up against Drogba and Nicolas Anelka.
Blackburn boss Sam Allardyce had gambled on him in the absence of Ryan Nelsen and Gael Givet.
That gamble may have seen rash as Drogba carved out an early chance for himself, dinking a neat curling shot just wide of the post despite being surrounded by a posse of defenders.
That was just a foretaste of what was to come as the Ivory Coast striker opened the scoring in the sixth minute with an almost effortless simplicity.
Anelka burst down the right, cut inside Jones and laid the ball back for Drogba to wrong-foot the entire Rovers defence and keeper Jason Brown with a neat left-footed finish back into the opposite corner.
Kalou threatened to further emphasise Chelsea's early dominance with a header from a corner that flew over the crossbar but from then on it was downhill for the visitors.
David Dunn won a free-kick in Morten Gamst Pedersen range after being halted by an agricultural challenge by Alex, but although the Norwegian was on target Chelsea keeper Ross Turnbull turned his effort behind.
Pedersen had another sight of goal but was wildly off-target with his volley, while Jones drew the loudest cheers from the Rovers fans so far with a series of crunching - but legal - challenges as Blackburn did their best to out-power the visitors.
Florent Malouda let fly from 20 yards out with a stinging drive but Brown managed to parry the shot, if somewhat unconvincingly.
Ancelotti was then forced to make a change just before the break with Branislav Ivanovic, who appeared to have been stood on unintentionally by El Hadji Diouf, limped off to be replaced by Zhirkov.
Blackburn, as if scenting that Chelsea may have missed a trick by scoring just a single goal in the first half, upped the tempo in the second.
Zhirkov's first involvement for Chelsea, right at the start of the second half was to head off the line after Chris Samba had beaten the flapping Turnbull to Pedersen's long throw.
Pedersen was then left cursing again when given space to shoot from 25 yards out but missing the target by some margin.
Frank Lampard, who had been having a quiet game by his standards, was much closer with a rising effort from the same range that was a whisker away from the top right-hand corner.
The suspicion that Chelsea had seemed over-confident in settling for a 1-0 win had been growing the longer the second half went on, and in the 70th minute Diouf made them pay.
The Senegal international hung in the air above Ferreira to meet Michel Salgado's cross with a superb header down into the corner.
Chelsea, stung in action, became more direct themselves and Alex struck a piledriver from a 30-yard free-kick over the bar.
With time running down, Drogba was denied a fine winner by a good block, then Jones crowned a magnificent debut with an intercepting header to keep out John Terry.
Rovers celebrated as though they had won the Premier League. Chelsea looked as though they had just lost it.