Afterwards Robson said: "It was shocking defending in the first 20 minutes which allowed Birmingham to take control.
"There was no platform in midfield for us to perform but now I realise why we are conceding so many goals."
Blues manager Steve Bruce said: "We started really well and gave us the springboard for the win.
"They couldn't handle big Emile Heskey, while the return to fitness of Clinton Morrison and David Dunn has been a major boost for us.
"Obviously, Bryan Robson has a big job on his hands but he has the resilience to come out on top. But he has a tough job on his hands."
It was the first time this season that Birmingham have scored more than two goals at home as they fully exploited their woefully weak opposition who look totally out of their depth in the Premiership.
As events transpired the success of Robbie Savage was pivotal to Birmingham's victory aspirations and the Welshman certainly responded in a positive mood when he confidently converted his fourth minute penalty.
It was the kind of start that Albion must have feared but they had no-one to blame but themselves as the punishment was self-inflicted.
A long clearance from goalkeeper Maik Taylor was headed on by Heskey to Morrison who was blatantly held back by his former colleague, Darren Purse.
Savage, the centre of much midweek speculation regarding his future at Birmingham, calmly slotted the ball low into the right-hand corner of the net with Russell Hoult diving in the opposite direction.
Albion, with Jason Koumas making his first start in 11 games, battled to get back into the game and gave warning of their determination when Zoltan Gera collected a flicked pass from Kanu to hit the post.
From the rebound Jonathan Greening managed to get in a powerful shot, but it was well covered by Taylor and Albion's hopes of an equaliser evaporated as Birmingham regained their composure to hit back at their local rivals.
The huge gaps in Albion's rearguard were evident for everyone to observe and Birmingham were not slow to exploit the situation as indicated when an anxious Purse was fortunate not to conceded a second penalty when Damien Johnson's cross was touched on by Heskey and Purse handled as he cleared his lines.
But there was not respite for Albion and in the 23rd minute Albion were ripped wide open following a free-kick. Darren Carter, celebrating his 21st birthday and his making his first Premiership start at St Andrew's, laid on a pass for Morrison to move around Neil Clement before drilling a low shot past Hoult.
Albion were now a complete shambles in defence and the powerful Heskey gathered a short pass from Dunn to notch Birmingham's third goal in the 30th minute with another powerful low drive.
Rob Earnshaw was potentially dangerous for Albion in the second half, but the Welsh international could not rescue a lost cause.
Albion's midfield was totally overrun and even Kanu could not make any impact as Birmingham dropped down a gear, before substitute Darren Anderton scored their only goal of the second period with a deflected free-kick in the 80th minute.
Man of the Match: David Dunn – The midfielder produced a top-flight performance and looks to be back to his best following a lengthy absence through injury.