Gareth Davies, Kristian Dacey and Dan Fish all scored as the Blues twice looked to mount comebacks.
But that was not enough to prevent them from a 22-34 defeat at Cardiff Arms Park.
The loss leaves the Blues a point behind the Dragons but the Newport region have two games in hand in a season where final standings could be crucial.
The Blues started brightly but when Davies’ clearance was charged down, it handed Leinster an early attacking scrum from which they scored after just four minutes.
The ball squirmed out at the side of the scrum, allowing No. 8 Jack Conan to wriggle over for a debut try. Jimmy Gopperth added the conversion.
The match continued at a furious pace with Leinster looking threatening every time they attacked. Richardt Strauss then broke through to score on 11 minutes, Gopperth adding the extras.
The Blues pushed the ball wide straight from kick-off and made good inroads. Davies jinking through the Leinster defence to give Cardiff a glimmer of hope.
Leinster then stretched their lead through Gopperth’s boot but with the score at 17-7, Copeland broke clean through. The chance was lost though when the Irishman declined to give the scoring pass.
However, the Blues rallied, exerting enough pressure to force Tom Denton into a place in the sin bin. The hosts won the resulting line out and hooker Dacey powered over for his fifth try of the campaign. Not bad for a front row forward.
Both outside halves traded penalties and then two more tries arrived for the visitors, securing the bonus point and top spot.
Fish scored a consolation try just days after he signed a new deal, keeping him at the region until 2016. But it wasn’t enough to stop Leinster going to the top of the table.
