Two tries and an assist by the virtually unknown Lee Williams ended a five-match losing streak for the Scarlets and guaranteed them a Heineken Cup spot next season thanks to a 30-9 win over Cardiff on Wednesday.
Williams, a 22-year-old scrum-half playing on the left wing, had shown his abilities as a member of Wales' World Cup-winning Sevens squad, but had never before scored in the Magners League.
This upset - the Blues' first home defeat of 2009 - condemns Dragons to be the lowest-placed Welsh region and they now face a European qualifying play-off in Italy.
It also frees five of the Scarlets to travel with Wales to North America, while the four Dragons chosen must miss out to help their region.
The Blues, without their five surviving Lions and key New Zealanders Paul Tito and Xavier Rush, nevertheless started strongly.
The hosts showed a willingness to throw the ball about, epitomised by a superb flick by Richie Rees to the looping Chris Czekaj.
They only had two Ben Blair penalties to show for it, however, when the Scarlets began to dictate, raking touchfinders from Daniel Evans and Rhys Priestland penning the Blues deep in their own half.
Their persistence was eventually rewarded with two Priestland penalties, sandwiching a first try for young Williams, who managed to get the ball down before the covering Rees dragged him in to touch-in-goal.
Blues' acting skipper Robin Sowden-Taylor was yellow-carded late in the first half and he was still absent when Williams side-stepped his way through the first line of defence before racing 50 yards and sending lock Dominic Day in at the flag.
The Blues sent on big kicking replacement Nicky Robinson to play the same territorial game and they cut the deficit with another Blair penalty.
The Scarlets introduced both their Lions, fly-half Stephen Jones and hooker Matthew Rees, as they tried to extend their lead, but it was two of the youngsters who created the killer try.
A thunderous tackle on Ceri Sweeney by centre Jonathan Davies saw the ball spill loose and Williams hared down the field to score under the posts.
When Darren Daniel clinched a bonus point with a breakaway try in injury time, the Scarlets' celebrations could really begin.
Pens: Blair 3
Tries: L Williams 2, Day, Daniel
Cons: S Jones 2
Pens: Priestland 2.
Blues: 15 Ben Blair, 14 Richard Mustoe, 13 Jamie Robinson, 12 Gareth Thomas, 11 Chris Czekaj, 10 Ceri Sweeney, 9 Richie Rees, 8 Maama Molitika, 7 Robin Sowden-Taylor, (capt) 6 Ben White, 5 Deiniol Jones, 4 Bradley Davies, 3 Faao Filise, 2 T Rhys Thomas, 1 John Yapp.
Replacements: 16 Gary Powell, 17 Ross Johnston, 18 James Down, 19 Scott Morgan, 20 Darren Allinson, 21 Dai Flanagan, 22 Nicky Robinson.
Scarlets: 15 Daniel Evans, 14 Darren Daniel, 13 Rob Higgitt, 12 Jon Davies, 11 Lee Williams, 10 Rhys Priestland, 9 Gavin Cattle, 8 David Lyons (capt), 7 Rob McCusker, 6 Dafydd Jones, 5 Dominic Day, 4 Lou Reed, 3 Deacon Manu, 2 Ken Owens, 1 Phil John.
Replacements: 16 Iestyn Thomas, 17 Matthew Rees, 18 Aaron Shingler, 19 Simon Easterby, 20 Martin Roberts, 21 Stephen Jones, 22 Ceiron Thomas.
Venue: Arms Park
Referee: James Jones (WRU)
Touch judges: Gwyn Morris (Wales), Hugh Watkins (Wales)
Television match official: Tony Rowlands (Wales)