The Scarlets secured a 38-23 win over the Blues at Parc y Scarlets but failed in their bid to make it into the semis.
Tries by wing Sean Lamont (two), prop Iestyn Thomas and number eight Ben Morgan had briefly put them in the play-offs, but the result in Italy halted their progress.
The Blues had gone into the match in pole position and despite taking the lead early into the second half, the hosts powered away after scoring 18 unanswered points.
The Scarlets dominated the opening minutes and took the lead when fly-half Stephen Jones booted a penalty.
However, the Blues hit back immediately. Despite having very little of the possession in the opening stages, stand-off Dan Parks put the visitors back on level terms.
Jones then regained the advantage for the Scarlets with another three pointer before the fly-half was tackled late by lock Michael Paterson, which cost Paterson 10 minutes in the sin-bin.
The Llanelli-based outfit decided to kick for touch instead of taking the penalty and it paid off when Thomas burrowed over from a driving maul, Jones adding the extras.
With so much on the line the Blues showed tremendous spirit and hit back with a converted try of their own through wing Chris Czekaj.
Parks then levelled matters with a penalty but Morgan then went over for the Scarlets, Jones converting, to retake the lead 20-13 in a topsy-turvy encounter.
But seven minutes before half-time, centre Dafydd Hewitt wasn't held in the tackle and subsequently drove to the line for a converted score.
Cardiff's Scottish stand-off seemed to have completed the comeback when he kicked a trademark drop-goal until substitute kicker Rhys Priestland responded with a penalty.
Lamont went over for the Scarlets before flanker Martyn Williams was sin-binned for the visitors. The wing then confirmed the victory when he finished off a fine move, Priestland converting.
Tries: Thomas, Morgan, Lamont 2
Con: Jones 2, Priestland
Pen: Jones 3, Priestland
For Cardiff Blues:
Tries: Czekaj, Hewitt
Con: Parks 2
Pen: Parks 2
Scarlets: 15 Morgan Stoddart, 14 George North, 13 Regan King, 12 Scott Williams, 11 Sean Lamont, 10 Stephen Jones, 9 Tavis Knoyle, 8 Ben Morgan, 7 Johnathan Edwards, 6 Rob McCusker, 5 Dominic Day, 4 Lou Reed, 3 Rhys Thomas, 2 Matthew Rees (c), 1 Iestyn Thomas.
Replacements: 16 Emyr Phillips, 17 Phil John, 18 Peter Edwards, 19 David Lyons, 20 Richie Pugh, 21 Martin Roberts, 22 Rhys Priestland, 23 Lee Williams.
Cardiff Blues: 15 Dan Fish, 14 Jamie Roberts, 13 Casey Laulala, 12 Dafydd Hewitt, 11 Chris Czekaj, 10 Dan Parks, 9 Richie Rees, 8 Xavier Rush, 7 Martyn Williams, 6 Sam Warburton, 5 Paul Tito (c), 4 Michael Paterson, 3 Taufa'ao Filise, 2 Gareth Williams, 1 Sam Hobbs.
Replacements: 16 Rhys Williams, 17 Tom Davies, 18 Scott Andrews, 19 Deiniol Jones, 20 Ma'ama Molitika, 21 Lloyd Williams, 22 Ceri Sweeney, 23 Harry Robinson.
Referee: Nigel Owens (WRU)
Assistant referees: Leighton Hodges (Wales), David Jones (Wales)