Gloucester pipped the Scarlets to the Pool Two title by virtue of scoring more tries in their four pool matches.
Nick Evans scored two tries in Harlequins's impressive opening but Gloucester scored five tries in their stunning fight back, including a brace for winger James Simpson-Daniel.
In the Parc Y Scarlets match on Saturday, three tries in the space of 12 second-half minutes, two from former All Blacks centre Casey Laulala, sank the Scarlets and took the Blues, the defending champions, into the quarter-finals as the only Welsh qualifier.
Northampton's 23-11 win over Leicester on Saturday in the East Midlands derby at Franklin's Gardens, their 11th in 12 games in all competitions, secured them a home semi-final.
First-half tries from Paul Diggin and scrum-half Alan Dickens, his first for the club, set up Northampton's win.
Centre Billy Twelvetrees scored his third try in two games for Leicester but they missed out on the semi-finals when Saracens claimed a losing bonus point against Newport Gwent Dragons on Sunday which allowed them to win Pool One.
Former South Africa fly-half Butch James made his first appearance in 10 months in the first half of Bath's 40-7 win over Sale at the Rec. James, who needed a knee reconstruction after being injured last April, helped to set up winger Michael Stephenson for a first half hat-trick of tries.
London Irish's semi-final challenge ended with a 19-9 defeat by Worcester at Sixways, where winger Chris Pennell scored the only try.
Wasps fly-half Dave Walder sank former club Newcastle with three penalties in a 9-3 win at Kingston Park between two sides who had no hope of progressing to the semi-finals.
On Sunday, the Ospreys recorded their first victory in the competition, but only just as Leeds fouthback to set up a tense finale. The Welsh side seemed to be cruising to victory after tries from Hanno Dirksen, Steve Tandy and Jamie Nutbrown helped them open up a 21-10 lead.
But a late rally saw Leeds earn a converted penalty try in injury time and make it a nervous few minutes for the home side, who had lost all their previous three games against Northampton, Bath and Gwent Dragons.
The semi finalists of the LV= Cup are ranked as follows:
1 Northampton Saints
2 Cardiff Blues
3 Gloucester Rugby
The semi finals will be played on Sunday, March 14 as follows:
Northampton Saints (1) v Saracens (4) at Franklin's Gardens
Cardiff Blues (2) v Gloucester Rugby (3) at Cardiff City Stadium
Kick off times TBC