El Karkouri hammered a stunning 25-yard free kick past Blackburn goalkeeper Brad Friedel to snatch all three points after Stephane Henchoz had been booked for holding back substitute Marcus Bent.
It looked like Charlton’s fierce endeavours would go unrewarded despite a plucky showing from a changed team following Saturday’s 2-1 defeat at Sheffield United.
Reed dropped Dennis Rommedahl, Darren Ambrose and Matt Holland but found a new hero in striker Lloyd Sam, who should have had a penalty when brought down by Lucas Neill in the first half.
He then missed a sitter from close range but was given a standing ovation when taken off 20 minutes from time.
Sam, 22, whose only previous start was against Hartlepool in the Carling Cup last season, was on loan at Sheffield Wednesday at the start of the campaign but is highly rated by Reed.
He took up a position just behind main striker Darren Bent in a modified version of the manager’s 4-5-1 formation, which had not entirely caught the imagination of the home fans but soon won their support as Sam posed a meaningful threat to the Blackburn goal.
Charlton were convinced he should have had a penalty when Neill’s clumsy-looking challenge from behind sent him tumbling in the 26th minute, but referee Chris Foy waved away the home appeals.
It was a big occasion for the Valley faithful, marking the 14th anniversary of their return to the famous old stadium after Charlton ground-shared with Crystal Palace and West Ham to save the club going bust.
The Addicks were seeking their first win in four games since squeaking through in a penalty shootout at Chesterfield to reach the quarter-finals of the Carling Cup for the first time.
Charlton suffered a setback when Djimi Traore had to be replaced by Souleymane Diawara in the 24th minute. But they redoubled their efforts and, following Bent’s run and cross from the left, Radostin Kishishev latched on to a return pass from Sam and curled his shot just wide from the edge of the area. Blackburn finished the half with Nonda forcing the keeper into a brave close-range save in stoppage-time.
McCarthy missed a good opportunity for Rovers early in the second half after Friedel’s brave save from Bryan Hughes at the other end.
The Dutch striker buried his shot in the side-netting when well placed for Shabani Nonda’s cross.
But Charlton fought bravely to retain the initiative and Hreidarsson missed a great chance, heading Andy Reid’s corner over the bar from six yards before the late drama.
CHARLTON: Carson, Young, El Karkouri, Hreidarsson, Traore (Diawara 26), Sam (Marcus Bent 71), Hughes, Faye, Kishishev (Holland 88), Reid, Darren Bent.
BLACKBURN: Friedel, Neill, Ooijer, Henchoz, Gray, Emerton (Peter 62), Savage, Bentley (Kerimoglu 46), Pedersen (Gallagher 46), Nonda, McCarthy.