Jamie Robinson scored the game's only try after just 11 minutes, with Ben Blair kicking a conversion and a penalty as the Blues ran out 10-6 winners at the Parc des Sports Aguilera.
Man-of-the-match Damien Traille and fly-half Julien Peyrelongue slotted a penalty apiece for the 2006 finalists but it wasn't enough to prevent the Blues from enhancing their chances of qualification for the quarter-finals for the second successive year.
The result was a far cry from the 2003 encounter between the two sides when a Biarritz side high on confidence crushed the Blues 75-25 in the first season of Welsh regional rugby.
This time around, the Blues never looked like being over-run. Instead, they were good value for their victory, thanks in large to some heroic defence and an ability to play their rugby in the right areas of the field.
Jamie Robinson's try arrived after former Wales skipper Gareth Thomas charged down Benjamin Thiery's attempted clearance, with Robinson reacting quickest to pounce on the loose ball and silence the 8,000-strong Biarritz faithful.
The Blues led 10-6 at the half-time interval and neither side were able to add to their tally in the second period. Biarritz pressed solidly for large periods but the Blues made tackle after tackle to ensure it is they who enter the final two games of the group stages with more to play for.