Report: Cardiff City 1-0 Blues
Jaden Philogene's first-half goal was the difference, although Birmingham City felt they should have been awarded a spot kick after the break when Scott Hogan was tripped.
The Head Coach made one change to the side that took to the field against Huddersfield Town in the last league game. Alfie Chang replaced Ryan Woods in the midfield, with George Hall returning to the matchday squad and Josh Williams' performance at Norwich City in the Carabao Cup earned him a spot amongst the substitutes.
The afternoon's first chance fell the way of Cardiff when Callum O'Dowda's low effort was blocked by Marc Roberts to the edge of the box, with Ryan Wintle sweeping the loose ball wide of the target.
From their next attack, the Bluebirds were more accurate and John Ruddy was called into action. He not only kept out Romaine Sawyers' low effort but denied Max Watters from point-blank range on the follow-up to produce a fine double save.
These moments came amidst a backdrop of contentious decisions going against the visitors and the annoyance, from a Blues perspective, was only heightened by Philogene's 17th-minute strike. He tapped in at the far post from an O'Dowda centre that found its way across the face of goal.
In response, Scott Hogan was a toe away from levelling after half an hour. Juninho Bacuna found Troy Deeney and his perfectly-weighted through ball split the defence of Steve Morison's team, only for a last-ditch tackle to stop the frontman from pulling the trigger.
There was then last gasp heroics at the other end when Max Watters wasted a chance one on one. Goalkeeper Ruddy won a battle of wits to save from the striker after he had raced clear.
HALF-TIME: Cardiff City 1-0 Blues
It was an end-to-end opening to the second 45 and both sides saw attempts blocked, with Blues' try coming as Jordan James set his sights from distance after O'Dowda's close-range stab was stopped by Marc Roberts.
Ruddy was then tested but again stood up to the challenge, keeping wise to O'Dowda's low shot and blocking it with his outstretched leg before a controversial moment occurred in the opposite penalty area.
Hogan blistered beyond Cedric Kipre and once in the box appeared to be tripped by the centre-back, however, referee Darren Bond and the nearside assistant referee determined there had been no foul to the total disbelief of nearly everyone in attendance.
Motivated by the sense of injustice, Eustace's men pressed on and Josh Williams' first league appearance from the bench added greater zip down the right flank, with Lukas Juktkiewicz, Jonathan Leko and Hall being added to the melting pot as the half went on.
Cardiff should have perhaps wrapped up the points as the clash approached the final ten minutes. Two substitutes linked up, but Kion Etete headed Mark Harris' dangerous cross wide - not that it mattered in terms of the final outcome.
The non-award of a penalty after Hogan was felled in the box could have dramatically shifted the contest in South Wales. Instead, it left players and fans alike scratching their heads in bewilderment.
Blues tasted defeat in the Welsh capital meaning the end of their unbeaten Championship start, as they remain on four points.
Cardiff City (4-2-3-1): Ryan Allsop, Mahlon Romeo, Perry NG, Cedric Kipre, Jamilu Collins, Andy Rinomhota, Ryan Wintle (captain), Callum O'Dowda (Sheyi Ojo 81), Romaine Sawyers (Robin Colwill 63), Jaden Philogene (Kion Etete 64), Max Watters (Mark Harris 46).
Substitutes not used: Jak Alnwick, Joe Ralls, Jack Simpson.
Booking: Ojo 90+1.
Blues (3-5-2): John Ruddy, Dion Sanderson, Marc Roberts (Josh Williams 55), Auston Trusty, Maxime Colin, Jordan James, Alfie Chang (George Hall 76), Juninho Bacuna (Jonathan Leko 76), Przemyslaw Placheta, Scott Hogan, Troy Deeney (Captain) (Lukas Jutkiewicz 72).
Substitutes not used: Neil Etheridge, George Friend, Jobe Bellingham.
Booking: Hogan 51, James 70, Trusty 90, Colin 90+4.
Referee: Darren Bond
Attendance: 18,893 (away 1,702)
The league journey continues back in B9, with Watford visiting on Tuesday night, kick-off 7.45pm. This is followed by another home outing as Wigan Athletic make the trip down the M6 in a week's time, kick-off 3pm. Tickets are on sale now for both fixtures - click here to buy!