Biarritz (6) 6
Pens: Traille, Peyrelongue.
Blues (10) 10
Try: J Robinson. Con: Blair. Pen: Blair.
Blues centre Jamie Robinson gets the touchdown for the game's crucial try
Cardiff Blues kept their unbeaten lead at the top of Pool Six with a gritty, attritional win away to Biarritz.
Jamie Robinson scored the decisive try of the game, on hand when Gareth Thomas charged down a Benjamin Thiery kick.
Ben Blair kicked the conversion and a penalty, but Damien Traille and Julien Peyrelongue landed penalties to see Biarritz trail just 10-6 at half-time.
There were no more scores after the break, although the Blues held out under huge pressure by Biarritz.
Running the ball was always going to prove a difficult option, with the Parc des Sports Aguilera pitch rendered heavy and sticky by the persistent rain that continued to drift in off the Atlantic.
The early exchanges were a mixture of midfield attrition and positional kicks, with Blues fly-half Nick Robinson edging his personal battle with opposite number Peyrelongue.
That proved just enough to swing the territorial stakes to allow Blair the game's first kick at goal, the Blues full-back stroking a beauty through the posts from 10 metres inside the Biarritz half.
The French side, needing a win to keep alive any realistic hopes of making the quarter-finals, then gifted their visitors a try.
We knew this was never going to be easy in France and the conditions meant it wouldn't be pretty
Blues director of rugby Dai Young
When Nick Robinson chipped a ball in behind the Biarritz defence from an attacking Blues scrum Thiery was alert to the danger, coming across to gather.
But the full-back then inexplicably shanked his clearing kick against the legs of the on-rushing Thomas, leaving centre Jamie Robinson to collect the rebound and go under the posts.
Blair added the simple conversion to give the Blues a handy 10-0 lead with just 12 minutes on the clock.
Nick Robinson tried to stretch the visitors' lead with a drop-goal when the Blues next got within range - a tactic that had worked well in the reverse fixture last week - but saw his effort go just wide.
Biarritz got a foothold in the game thanks to the efforts of Thiery, whose whole-hearted chase of his own up-and-under enticed the Blues tacklers to go off their feet at the breakdown.
That earned a long-range penalty from Traille that just had the legs to drop over the crossbar.
A second Biarritz penalty - Peyrelongue converting this closer chance - soon followed as Nick Robinson had a kick charged down, forcing Leigh Halfpenny to concede a penalty as the wing scrambled back to cover.
Robinson's wayward kicking soon put his team-mates under pressure again as the Blues fly-half sliced one so badly that it went backwards.
Another penalty was conceded under Biarritz pressure, but this time Peyrelongue's kick from the right rebounded off the near post to be gratefully gathered by Blues hooker Gareth Williams.
That meant the Blues still held, somewhat fortuitously, a 10-6 lead at half-time.
Biarritz opted to take their Welsh visitors on up front in the second half, a sensible option on a pitch that was cutting up badly as the game progressed.
A series of drives took the French to within a sniff of a try, only prevented by Martyn Williams burrowing underneath Eduard Coetzee to hold up the ball as the Biarritz prop was shoved over the whitewash.
That typically clever play from the Blues open-side was the centre piece of an immense defensive effort, as Biarritz put the Welsh region under the cosh for a gruelling 15-minute period.
But the Blues held firm, even surviving another charge-down from Nick Robinson's wayward boot.
Having weathered the Biarritz storm, the Blues lifted the siege and patiently worked their way upfield away from the danger zone of their own line.
Although the visitors were unable to find another score to give them any sort of a cushion, crucially they were able to cancel out any French threat.
Halfpenny, Andy Powell and replacement wing Tom James all made important breaks to keep Biarritz on the back foot and end their Heineken Cup hopes, despite a bonus point in defeat.
Pool 6 is now a two-horse race between the Blues and Gloucester, with the clash between the two at Kingsholm in the next round in mid-January now boiling up nicely.
Blues director of rugby Dai Young:
"We're not going to start talking about the quarter-finals with two big games remaining, but we are very pleased with this result.
"I'm proud of the character the boys showed to pull off this win. We knew this was never going to be easy in France and the conditions meant it wouldn't be pretty.
"But this was all about who wanted it most and which team was prepared to put their bodies on the line for the win."
Biarritz coach John Isaac:
"We're very disappointed with the result.
"We had most of the ball and territory but - just like last week against Cardiff - we did not make it count on the scoreboard."
Biarritz: Benjamin Thiery; Ilikena Bolakoro, Philippe Bidabe, Damien Traille, Takudzwa Ngwenya; Julien Peyrelongue, Fabien Cibray; Eduard Coetzee, Benjamin Noirot, Mosese Moala, Jerome Thion (capt), Trevor Hall, Mohammed Dridi, Wenceslas Lauret, Imanol Harinordoquy.
Replacements: Benoit August for Noirot (55), Benoit Bourrust for Moala (55), Cesar Damiani for Hall (63), Samiu Vahafolau for Harinordoquy (57), Peio Som for Lauret (55), Romain Cabannes for Ngwenya (68), Laurent Tranier for Cibray (55).
Cardiff Blues: Ben Blair; Leigh Halfpenny, Jamie Robinson, Tom Shanklin, Gareth Thomas; Nick Robinson, Jason Spice; Gethin Jenkins, Gareth Williams, Gary Powell, Deiniol Jones, Paul Tito (capt), Ma'ama Molitika, Martyn Williams, Andy Powell.
Replacements: Taufa'ao Filise for G. Williams (60), T Rhys Thomas for G. Powell (60), Scott Morgan, Robin Sowden-Taylor, Darren Allinson, Ceri Sweeney, Tom James for Halfpenny (66).
Referee: Peter Fitzgibbon (Ireland).