Larsson's injury-time strike completed a rousing performance from the Blues against Neil Warnock's line-up which showed 11 changes to the team beaten by Everton in the Premiership on Saturday.
Nicklas Bendtner and Cameron Jerome had both beaten the Blades offside trap to fire their side into a match-winning lead which will do much to strengthen Bruce's hold on his St Andrews post despite their Championship promotion bid faltering recently.
It was an awful second half showing from Warnock's men who had started in promising fashion and looked a good bet for victory after Ade Akinbiyi's 21st-minute opener.
Danny Webber tested visiting keeper Colin Doyle with a good low shot and youngster Nicky Law almost capped his second appearance with a goal in the 19th minute when his free-kick was flapped away.
But it was Akinbiyi who broke the deadlock with his first goal since February when Webber crossed from the right and the big striker scooped his shot over Doyle to give the home side the lead.
Birmingham had their own chances in an open game with Dudley Campbell catching Craig Short in possession in the 32nd minute before finding Jerome who shot straight at Ian Bennett in goal.
Immediately at the other end Paul Ifill was played in by Akinbiyi but elected to turn and shoot wide rather than pass to Steve Kabba, who was racing unmarked into the box.
But Birmingham equalised out of nowhere four minutes before the break when Blades captain Chris Morgan hammered an attempted clearance straight at Campbell and the ball squirmed up and over Bennett.
Law came close to putting the Blades back in front just after the break with a rising right-foot effort while David Unsworth let fly with a left-foot effort which was tipped over the bar.
Two minutes later Julian Gray played in Campbell at the other end but Bennett got a hand to his low shot and managed to divert it onto the base of the post.
Immediately the Blades counter-attacked and Akinbiyi roared free down the right flank but under pressure from Blues defender Mathew Sadler the front man fired his eventual shot high and wide.
Substitute Christian Nade almost made an immediate impression in the 68th minute when he wriggled through the Blues defence and shot inches wide of Doyle's right-hand post.
But it would be another substitute in Bendtner - a 58th-minute replacement for Campbell - who turned the game by springing the offside trap to put his side ahead in the 71st minute.
It looked all over 12 minutes from time when Jerome came up with almost a carbon copy of his team-mate's effort, racing clear and rounding the advancing Bennett to put his side 3-1 up.
There was still life in the Premiership side who gave themselves hope in the 83rd minute when Nick Montgomery lashed home a low shot from 25 yards to reduce the deficit.
But it was finally all over deep into injury time when Larsson nudged home the simplest of chances to give the travelling Blues fans an overdue cause for celebration.