Northampton maintained their perfect Heineken Cup record with a 23-19 victory at the Cardiff, after the Blues had been hit by the second-half dismissal of Xavier Rush.
Substitute Calum Clark crossed over for the decisive score, and his side's only try, seven minutes from the end to make the Blues pay for Rush's red card for a dangerous tackle on Courtney Lawes.
The Blues had looked like they might hold on, with Richard Mustoe's try just after the break plus 14 points from the boot of Dan Parks rewarding their endeavour. But it was the Saints' pack that again ultimately decided the contest, with Tongan prop Soane Tonga'uiha deservedly named man of the match, while Steve Myler's six penalties kept the scoreboard ticking over.
It meant a first home loss in 13 matches in the competition for the Welshmen, who again failed to come to grips with the Saints pack after their dominance at Franklin's Gardens last time out. Northampton were therefore naturally keen to have another go up front, and spurned an early free-kick for the put-in at the scrum. Their forwards duly drove the Blues back, the home side were penalised and Myler kicked the visitors into a third-minute lead.
But the Blues showed more flair behind and their pressure was eventually rewarded with a simple Parks penalty for offside, only for Myler to restore the Saints' lead when Blues wing Richard Mustoe was penalised for holding on.
The Welsh region kept pounding away, the lively Martyn Williams scavenging for possession, and Bradley Davies and Rush were both held up before the Saints transgressed again, centre James Downey receiving a yellow card and Parks slotting over the penalty.
The Scottish fly-half added another three points when Northampton flanker Tom Wood offended at the tackle, but on a rare visit down field Myler rounded off the opening period with another simple penalty.
The Blues introduced British Lion Jamie Roberts at half-time for his first outing of the season following a wrist operation in the summer and the burley centre played a typically muscular part in the game's first try. A mazy run by fellow centre Casey Laulala set up the move, which was finished off by winger Mustoe, twisting out of a tackle to touch down, Parks converting.
Blues lost lock Davies to the sin bin for slowing down ball after the breakdown and Myler kicked his fourth penalty to cut the deficit.
The Saints were beginning to threaten more with ball in hand and a run from deep by England full-back Ben Foden saw the move ended by a Rush tackle on Courtney Lawes which referee Jerome Garces thought dangerous enough to justify a red card. Myler inevitably kicked the resultant penalty.
When Northampton lock Christian Day flattened home scrum-half Richie Rees there were loud demands from the home section of the 7,000-plus crowd for similar treatment, but the French official saw nothing amiss. Another penalty conceded by the beleaguered Blues scrum allowed Myler to put the English team in front, but the rivals refused to capitulate and a snap drop goal by Parks edged them back into the lead.
It was mainly a rearguard action by the Welshmen, however, and after a superb tackle by Laulala had held up Wood, the rest of the Northampton back drove over the line, substitute Clark eventually being credited with a try after lengthy deliberations by the television adjudicator. Myler, for the first time, missed the kick.
A yellow card for Saints' Clark evened the numbers, but a knock on by flanker Ma'ama Molitika in the opposing 22 saw the Blues' last hopes disappear.