Flood combined with Michael Chopra in an 84th minute break that proved the only notable action of the second half before sliding the ball under home keeper Tony Warner.
Blackwell has now seen his side pick up only four points from a possible 12 in their first four Championship outings and that is not good enough for a side harbouring serious promotion ambitions.
Leeds' misfiring attack drew another blank as Cardiff City deservedly took the points.
The Whites have now failed to score from open play in three of their four matches this season - another worrying trend all too familiar from last year.
Blackwell will argue that his side showed promise and urgency in the final quarter but United fans could be forgiven for asking for that from the start as they watched another smash-and-grab at Elland Road.
Stephen McPhail made his first return to Leeds but it was the activity in the Directors' Box that attracted most of the pre-match attention.
Peter Ridsdale chatted amongst his colleagues before kick-off on only his second visit to the club his regime almost destroyed, but there was little familiar about the surroundings.
Since Ridsdale's departure Leeds have dropped a division and play in front of a half-empty stadium - a far cry from the Champions League adventure his band of merry men led the Whites on.
And Ridsdale's presence was by far the most exciting part of another dour first half.
Neither side managed a shot on target until two long-range efforts on the half-hour mark forced Warner into a couple of comfortable saves.
Chopra's effort drifted straight at Warner before McPhail's 25-yard free-kick at least made the Leeds keeper shuffle a couple of steps to his right.
Matthew Kilgallon should have opened the scoring earlier on but his free header had flown high over the target after he got on the end of Eddie Lewis' free-kick.
That was to prove Leeds' most threatening foray into the Cardiff half until the 43rd minute, when Lewis' corner created a manic goalmouth scramble that saw Kilgallon, substitute Eirik Bakke and Shaun Derry all see their close-range efforts blocked by a string of defenders.
Bakke had replaced Steve Stone ten minutes earlier, with the former England flyer clearly feeling the effects of his groin strain.
The game looked to be heading towards a 0-0 draw before Flood broke the deadlock and sent the visiting fans into raptures near the end of a miserable second half.
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