Scarlets (17) 23
Tries: Priestland, L. Williams, Lyons. Cons: Priestland. Pens: Priestland 2.
Blues (7) 38
Tries: Shanklin, Laulala 2, Filise, Tito. Cons: Blair 5. Pens: Blair.
Cardiff Blues booked a home semi-final place in the LV= Cup against Gloucester with a five-try win at the Scarlets, who just failed to advance themselves.
Gloucester overhauled the Scarlets in Pool 2 with their five-try 31-29 bonus-point win at Harlequins on Sunday.
Scarlets led 17-7 at half-time with tries from Rhys Priestland and Lee Williams, but Blues then took control.
Centre Casey Laulala scored two of the Blues' four second-half tries, as the Scarlets saw their hard work undone.
Scarlets: Daniel Evans; Lee Williams, Gareth Maule, Regan King, Andy Fenby; Rhys Priestland, Tavis Knoyle; Phil John, Ken Owens, Deacon Manu, Lou Reed, Damian Welch, Rob McCusker, Josh Turnbull, David Lyons (capt).
Replacements: Iestyn Thomas for Manu (57), Emyr Phillips, Rhys Thomas for Maule (57), Vernon Cooper for Reed (61), Johnathan Edwards for Turnbull (57), Martin Roberts, Daniel Newton for King (72), Joe Ajuwa.
Blues: Ben Blair; Gareth Thomas, Casey Laulala, Tom Shanklin, Dafydd James; Ceri Sweeney, Darren Allinson; Taufa'ao Filise, T Rhys Thomas, Gary Powell, Deiniol Jones, Paul Tito (capt), Maama Molitika, Sam Warburton, Xavier Rush.
Replacements: Rhys James, Scott Andrews for G. Powell (79), Ian George, Scott Morgan for Warburton (70), Andries Pretorius for Molitika (64), Tom Slater, Sam Norton-Knight, Chris Czekaj.
Referee: Greg Garner (RFU).