Blues 27 (13)
Marc Stcherbina crosses for Cardiff Blues' fourth try for a bonus point
Tries: Sowden-Taylor, G Williams, R Williams, Stcherbina.
Cons: N Robinson 2.
Leinster 11 (6)
Pens: Contepomi 2.
Cardiff Blues ended Leinster's Magners League title hopes with a bonus-point win at the Arms Park to move above the Irish and briefly go top of the table.
But the result left the door open for the Ospreys, who became champions on Saturday with a win at Border Reivers.
Robin Sowden-Taylor, Gareth Williams, Rhys Williams and Marc Stcherbina all crossed in a dominant Blues display.
Leinster never flowed on a muddy pitch, managing a Shane Horgan try and six points from Felipe Contepomi's boot.
The Irish entered the match knowing that a win plus a bonus point for scoring at least four tries would secure the title, regardless of how the Ospreys performed in Scotland.
But any thoughts that Leinster had of taking the game by the scruff of the neck were undone within six minutes as Sowden-Taylor put the Blues ahead.
Nick Robinson made the crucial break, accelerating past a surprised Keith Gleeson and then timing his pass to Sowden-Taylor to perfection.
The open-side stepped smartly inside wing Denis Hickie to cross the Leinster line, although Ben Blair missed the conversion.
Contepomi, who earlier in the day heard he had qualified as a doctor, got the Irish on the scoreboard with a brace of penalties, bracketing one from Blair for the home side.
The heavy rain over the past few days had made the pitch sticky and the handling sometimes unsure.
So it was little surprise that the next score was a forward effort, as the Blues opted for a five-metre line-out to maul hooker Gareth Williams over for an unconverted try.
That saw the Blues take a 13-6 lead into the second half and leave Leinster's title hopes teetering on the edge.
The picture for the Irish got even worse as the Blues produced two quick tries to boost their own pursuit of the Celtic crown.
The first was engineered by Nick Robinson whose fine cross-kick was fielded by Chris Czekaj on the right to send full-back Rhys Williams over, Robinson converting.
It's two years in a row now we've lost the title on the last weekend
Leinster coach Michael Cheika
Then Stcherbina was the beneficiary of a muffed chip from Horgan.
The kick failed to clear the Australian centre, was gathered and then deposited over the line for another seven-pointer that also earned a precious bonus point.
Horgan made amends on 55 minutes when his grubber was miss-fielded by Rhys Williams, allowing the Ireland back to hack on and touch down.
But Contepomi missed the conversion and then found himself sin-binned after taking out a jumping Jamie Robinson in mid-air.
Leinster survived the 10 minutes a man down without shipping any more points, but they needed more than just survival.
Hickie and Gordon D'Arcy continued to make ground, but any breaks the pacy duo made either petered out through a lack of support or were ended by handling errors as the visitors tried to force the pace.
The Blues exited the field knowing they had done everything in their power for a first Magners League title.
But events the following day at Netherdale saw the title slip through the Blues' fingers as their fierce Welsh rivals the Ospreys pip them to the title by a point after beating Borders.
Blues coach Dai Young:
"The players went out from the word go and really performed, so we've done as much as we can.
"Hopefully it will be enough but it's out of our hands now, hopefully Borders can do us a favour.
"It's the last game ever for them so I'm sure there will be a lot of emotion up there, lets hope that the Ospreys freeze."
Leinster coach Michael Cheika:
"It's two years in a row now we've lost the title on the last weekend and it's very disappointing.
"We weren't good enough tonight, it's pretty clear, although we tried very hard and there was a lot of commitment.
"But it didn't flow for us, we made a few mistakes and that took the pressure off them despite us having a lot of the ball."
Blues: Ben Blair; Rhys Williams, Jamie Robinson, Marc Stcherbina, Chris Czekaj; Nick Robinson, Mike Phillips; Gethin Jenkins, Gareth Williams, Gary Powell, Deiniol Jones, Robert Sidoli, Mark Lewis, Robin Sowden-Taylor, Xavier Rush (capt).
Replacements: Scott Roberts for Jenkins (65), T. Rhys Thomas for G. Williams (73), Scott Morgan for Lewis (69), Rhys Shellard for Sowden-Taylor (73), Wayne Evans for Phillips (74), Nick Macleod for Blair (69), Tom James for R. Williams (65).
Leinster: Girvan Dempsey; Rob Kearney, Shane Horgan, Gordon D'Arcy, Denis Hickie; Felipe Contepomi (capt), Chris Whitaker; Reggie Corrigan, Bernard Jackman, Ross McCarron, Trevor Hogan, Malcolm O'Kelly, Stephen Keogh, Keith Gleeson, Jamie Heaslip.
Replacements: Stan Wright for Corrigan (41), Brian Blaney, Owen Finegan for O'Kelly (63), Cameron Jowitt, Guy Easterby for Whitaker (41), Christian Warner for Kearney (56), Luke Fitzgerald for Dempsey (67).
Referee: Malcolm Changleng (SRU).