Blues progressed to the third round of the Carling Cup at St. Andrew's this evening but were made to work hard for their victory by the plucky underdogs of Rochdale.
The visitors took a shock lead through Gary Jones and only a dubious penalty converted by James McFadden sent the top flight hosts in level at the break.
Two early second half goals from Alex McLeish's side by the returning David Murphy and full debutant Matt Derbyshire proved enough to complete the job.
Although a second goal from Jones made an uncomfortable final few minutes for the Barclays Premier League side.
Enric Valles made his Blues bow and there was also a first start for Derbyshire whilst Murphy was handed his first appearance in 16 months following a long spell on the sidelines due to injury.
McLeish has made eight changes in total from the side that beat Blackburn in the Barclays Premier League on Saturday.
Maik Taylor lined up between the sticks with Keith Fahey filling in at right-back and a chance for Spanish midfielder Michel to stake his claim.
Blues survived an early scare when Taylor's clearance deflected into the path of Andrew Tutte and fortunately the on-loan Manchester City midfielder's goalbound shot was blocked by the head of Murphy.
The hosts went close when Lee Bowyer latched onto James McFadden's pass and, from a narrow angle, the former West Ham man clipped the ball over visiting keeper Josh Lillis but saw his shot strike the outside of the post.
Derbyshire then attempted an audacious overhead kick that was easily held by Lillis.
Dale came within a whisker of taking a shock lead when former Kettering Town striker Anthony Elding headed just past the post from a left wing cross.
Last season's 22-goal man Chris O'Grady went even closer when he showed great control to skip past Scott Dann and fire in a low shot that skidded inches wide of Taylor's left-hand post.
Blues didn't heed the warning and on 25 minutes it was the League One side that opened the scoring through Gary Jones.
Following a flowing breakaway move down Blues' left flank, the ball was pulled back for right-back Scott Wiseman, who had made his way up into the area.
Although Taylor was equal to the effort he could only parry away and the captain fired home from close range.
The visitors' lead lasted just three minutes but the equaliser came in controversial circumstances from a harshly awarded penalty.
As Derbyshire and Marcus Holness tussled for the ball inside the area, the Blues striker went to ground after the slightest of nudges from the Dale defender.
The referee was unmoved but it was his assistant Nigel Murphy that signalled for the infringement and, from the resultant spot kick, McFadden fired an unstoppable shot into the top left corner.
The scorer was looking the liveliest of Blues' offensive players and the Scot hit a low shot soon afterwards that was saved low down by Lillis.
The Barclays Premier League side showed a glimpse of their top flight credentials just before the break with a delightful flowing move.
McFadden was the main architect and it concluded with Fahey whipping in a tremendous low cross that just evaded Derbyshire for what would have been a certain goal.
Blues were forced into a change at the interval as Seb Larsson replaced McFadden, who had taken a knock and was taken off as a precaution ahead of the weekend trip to Bolton.
A bright start to the second half from the visitors concluded with Rochdale's highly-rated centre-half trying his luck with an ambitious overhead kick that never looked like troubling Taylor.
Moments later at the other end it was another defender taking centre stage with Murphy the man of the moment.
Johnson headed a Larsson corner back across goal and the left-back was in the right place to hook the ball past Lillis.
And just five minutes later the tie was all-but over as Blues opened up a two-goal advantage with Bowyer's measured ball splitting the Dale defence and putting Larsson in on the right side of the box.
The Swede's low cross was fumbled by Lillis and as the ball squirmed out of the keeper's grasp it was Derbyshire who was there to take advantage to mark his first start with a goal.
The ball just crossed the line despite the frantic efforts of Elding to clear.
Blues were in cruise control from that moment on and created several chances to extend their lead.
Derbyshire fired into the side-netting from the right side of the box before being replaced by Nikola Zigic and leaving the field to warm applause from the home fans.
Zigic used his height to good effect to nod the ball down to O'Connor whose first-time effort veered well wide of goal.
The same can be said of Elding's shot at the other end when the Dale number nine got his shot completely wrong when well placed on the six yard box from O'Grady's pull back.
And with 15 minutes to go it was more good work from O'Grady that set up Jones inside the area and the skipper slotted home coolly to put the game back on a knife -edge again.
Blues then made their final sub and it was an historic moment as Nathan Redmond became the second youngest ever player to play in the Blues first team with the 16-year-old replacing Valles.
Just 34 days older than Trevor Francis when the legendary Blues striker made his bow, Redmond made an immediate impression with a vicious shot from distance that fizzed just over the bar.
The livewire teenager saw plenty of the ball during an impressive cameo performance to give plenty of hope for the future.
For the time being the focus returns to league matters and Blues prepare for a trip to the Reebok Stadium on Sunday.
Blues (4-4-2): Maik Taylor; Keith Fahey, David Murphy, Scott Dann, Roger Johnson; Enric Valles (Nathan Redmond 76), Lee Bowyer, Michel, James McFadden (Seb Larsson 46); Matt Derbyshire (Nikola Zigic 65), Garry O'Connor.
Subs (not used): Ben Foster, Craig Gardner, Cameron Jerome, Barry Ferguson.
Goals: McFadden (28 pen), Murphy (48), Derbyshire (53)
Rochdale (4-4-2): Josh Lillis, Scott Wiseman, Joe Widdowson, Marcus Holness (Josh Thompson 63), Brian Barry-Murphy, Jason Kennedy, Gary Jones, Anthony Elding (Jean-Louis Akpa Akpro 63), Andrew Tutte (Matt Done 63), Chris O'Grady, Craig Dawson.
Subs (from): Matt Edwards, Jack Redshaw, Alan Goodall, Helio Andre.
Goal: Jones (24, 75)
Attendance: 6,431 (598 away fans)