London Irish must rely on Worcester beating Saracens on Saturday if they are to reach the semi-finals of the LV= Cup after the Blues upset their momentum at a damp Arms Park.
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And it was largely down to their own mistakes that the visitors lost.
Irish never looked like a team that had won all five games in January, even if a muddy pitch and gusting wind contributed to an error-strewn performance.
The visitors arrived in Wales with qualification in their own hands, leading their pool by a point from Saracens, while the Blues, although theoretically capable of reaching the last four, were realistically playing for pride only.
With nine players away with the senior Wales squad and a handful of others in international under-20 action, the Blues were further weakened after six minutes when lock Cory Hill limped off, flanker Michael Paterson having to move forward in the pack.
But the New Zealander was still a force in the loose, breaking downfield, aided by Rhys Patchell, before a move broke down in the Irish 22.
The Reading-based side broke the deadlock, prolific full-back Tom Homer slotting over a penalty from straight in front midway through the first half.
Promising Blues fly-half Patchell, released from national age-group duty, darted down the right, but was beaten to his kick ahead by Homer, who hacked the ball into touch five yards from his goal-line.
The Blues kept up the pressure and the Irish defence eventually conceded a penalty, only for Patchell to push it wide.
But the home side were soon on the scoreboard. Homer fumbled a Patchell grubber on his own line and Blues acting captain Gavin Evans pounced on the loose ball to touch down. Patchell converted.
Homer's bad luck continued when he saw a 40-yard penalty rebound from a post shortly before the interval.
The Blues emerged in clean jerseys, but were soon getting them dirty in defence as the visitors showed a new urgency, epitomised by a scything break from centre Guy Armitage, only for the attack to end tamely with a knock-on.
The Welsh region, struggling at scrum time, brought on new props, but they were promptly penalised at the next set-piece and Homer cut the deficit to a single point.
It sparked the Blues into life, however, with try-scorer Evans and scrum-half Alex Walker leading an attack which ended in a penalty, which Patchell missed.
The number 10 made up for it shortly afterwards when he was offered a slightly easier chance.
With a try enough to make a semi-final spot almost certain, the visitors swarmed into the Blues 22 and turned down a kickable penalty to go for the lineout, only to spill the ball as they tried to force their way over.