Blues (9) 19 Try: Mustoe Con: Parks Pens: Parks (3) DG: Parks Northampton (9) 23 Try: Clark Pens: Myler (6)
Rush looks back ruefully after receiving his red card
Xavier Rush's second-half red card was the turning point as Northampton took control of Heineken Cup Pool One and left the Blues' ambitions in tatters.
Steve Myler and Dan Parks traded kick for kick in a brutal game, before Blues wing Richard Mustoe got the first try.
But in a match of three sin-binnings Rush saw a straight red for a scything head-high tackle on Courtney Lawes.
Scrum dominance and replacement Calum Clark's decisive try saw the Saints home, despite Clark's late yellow card.
With Castres' game in Edinburgh delayed until Monday, Northampton now head the group with 16 points from the French side (9) and the Blues (6).
After Northampton's feisty 23-15 win at Franklin's Gardens in last week's reverse fixture, the two sides were quickly at each other's throats in a frosty Cardiff City Stadium.
A first-minute scrum popped up prompting a mass scuffle, Northampton then awarded two penalties as warring opposing hookers Gareth Williams and Dylan Hartley again came up and across from scrums.
Myler opted to kick for goal after the second penalty, the fly-half making no mistake to open the scoring.
The Blues enjoyed a good period of territory and possession, but blew their best try-scoring chance from turnover possession in the 22 as Parks' miss-pass squandered an overlap.
Jamie Roberts came off the bench for his first game in six months
The home side's 15 minutes of dominance eventually told on the scoreboard as Parks levelled the game with a penalty from in front of the posts.
Myler nudged the Saints back ahead with a difficult penalty after Mustoe was isolated at a ruck.
In contrast the Blues chose to boot two difficult penalty chances to touch, the second leading to a sustained assault on the Northampton line that saw Saints centre James Downey sent to the sin-bin for killing the ball.
More fisticuffs burst out, Paul Tito heavily engaged with Hartley and opposing locks Lawes and Christian Day, before Parks slotted over the penalty to level the match at 6-6 on the half-hour mark.
The Blues fired back at the 14-man Saints, Parks putting his side ahead for the first time after another ruck offence inside the Northampton 22.
Tom James was the next to clash with the ever-spikey Hartley, but it was the Northampton front row's dominance at the scrum - man-of-the-match Soane Tonga'uiha outstanding - that helped the depleted visitors secure another penalty for Myler to level the game at the break.
The Blues brought Jamie Roberts off the bench in place of Tom Shanklin for the start of the second half, the Wales star returning from six months out with a wrist injury.
It was his centre partner Casey Laulala who lit the Blues' touch paper, though, splitting the Saints midfield defence on halfway, the attack carried forward for Mustoe to pile over from close range.
Parks calmly slotted the conversion as both sides were warned for another brawl, Martyn Williams and Phil Dowson at the heart of it.
Blues lock Bradley Davies was foolishly sin-binned, sticking out a stray boot when he was on the floor in a ruck to disrupt Northampton ball.
Myler kicked the resultant penalty, before a scintillating Ben Foden break again put the Blues on the back foot.
Paul Tito gets to grips with Dylan Hartley in a bad-tempered game
The ball was sent across the line to Lawes who was felled by a high, flying tackle from Rush, referee Jerome Garces having little choice but to show the Blues talisman a straight red.
Myler punished the 13-man Blues with the resultant penalty to reduce the gap to 16-15, the home side then holding on until Davies' return from the bin.
Northampton's dominant scrum won yet another penalty to give Myler his sixth successful kick, but that was cancelled out as a strong drive from Roberts in midfield gave Parks the perfect platform for a drop-goal.
The 14-man Blues defended heroically, winning crucial ball at the breakdown, but the pressure seemed sure to tell and replacement Clark was controversially awarded a short-range try by the video referee.
Myler failed with the conversion to end his 100% kicking record, then the playing numbers were levelled up three minutes from time as try-scorer Clark was sin-binned for killing the ball at a ruck.
Parks kicked to touch in the Saints' 22, but the attack broke down after a Maama Molitika knock-on.
That allowed the Saints to run down the clock, maintaining their 100% record and inflicting on the Blues a first home loss in 13 matches in the competition.
Blues coach Dai Young: "[The red card and Clark try] were certainly talking points in the tunnel, but the referee has made his decisions and there's no point in me saying anything.
"We had enough opportunities in the first half to get the scoreboard ticking and didn't take them.
"We scored our try early in the second period, but then had a yellow card and the sending-off and the whole game changed.
"We've got two tough matches left and probably need to win both to get into the Amlin Cup, so that's what we'll be trying to do."
Northampton coach Jim Mallinder: "Last year we lost at Munster and Perpignan and this showed we've learned how to win those big games.
"It was very physical, quite confrontational at times, as we expected. In the first half we came under severe pressure and were quite fortunate to go in level at the interval.
"But I was very pleased with the way we came out for the second half and our pack got on top, which made the difference.
"You always want more. In the first half we didn't look after the ball well and our kicking game was poor.
"In the second half we kicked better, they had to kick back and allowed our back three to come into things, with Ben Foden showing up well.
"Now it's a matter of seeing the pool out and hopefully getting a home draw in the quarter-finals."
Blues: Czekaj, Mustoe, Laulala, Shanklin, James, Parks, Rees, Jenkins, G. Williams, Filise, B. Davies, Tito, Molitika, M. Williams, Rush. Replacements: Roberts for Shanklin (40), R. Williams for G. Williams (58), Yapp for Filise (57), Paterson for Tito (58), Warburton for M. Williams (58). Not Used: Hobbs, Slater, Sweeney.
Sin Bin: B. Davies (48).
Sent Off: Rush (53).
Northampton: Foden, Ashton, Clarke, Downey, Diggin, Myler, Powell, Tonga'uiha, Hartley, Mujati, Lawes, Day, Dowson, Wood, Wilson. Replacements: Dickson for Powell (47), Clark for Lawes (52), Sorenson for Day (76). Not Used: Sharman, Dreyer, Waller, Geraghty, Ansbro.
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