Further tries from Gareth Davies and Dominic Day helped the Scarlets win an error-strewn game in Cardiff as Nigel Davies' side ensured their play-off hopes were boosted while the Blues' took a hit.
For the home side it was returning Wales wing Alex Cuthbert and Casey Laulala who crossed for scores but it wasn't enough to stop their Welsh rivals.
The first genuine chance came from the Blues with a typically languid run by centre Laulala.
Fed by Blues' braided No. 7 Josh Navidi, Laulala stepped inside the Scarlets' drift defence and hared straight for posts.
Eventually three Scarlets defenders brought him down but rather than a warning shot, it was a highlight of the home side's limited attack in the first half.
In a tight derby it was a mistake, by Cuthbert, which led to the first try for visitors wing Fenby.
After a counter-attack fizzled out, another Wales Grand Slam hero making his domestic return Wales centre Jonathan Davies kicked deep into the Blues' 22.
The covering Cuthbert was being hounded by Scarlets captain Gareth Maule and let the centre's presence get to him.
When reaching down to collect the ball Maule shoved him out the way, giving the trailing Fenby the easiest of tries. Fly-half Aled Thomas converted from the touchline to make it 7-0 at half-time.
Two minutes after the break Fenby was at it again, but this time he didn't need Maule as his foil.
Fenby turned over a ruck in the Blues' 22 and kicked on. Parks was the only man back for the Blues and tried to side-foot the ball out of play but missed completely and the incredulous Fenby had another carbon copy try to his name which Thomas converted.
The Blues shook themselves down to put some phases together and after a quick tap and go by centre Dafydd Hewitt gave them quick ball, Scarlets wing Sean Lamont deliberately knocked on. Referee Nigel Owens had no hesitation in sin-binning the Scot and in his absence the home side got one try back.
With the Scarlets in possession, Cuthbert picked off a pass by Thomas and ran it under the sticks from 60 metres. Ceri Sweeney converted and the Blues looked odds on for the next score.
But the Scarlets hit them on the counter-attack. From a ruck Lamont went blind and slipped the ball to scrum-half Gareth Davies to score.
This time Thomas missed from the touchline.
The Blues weren't done though and when Laulala popped up at fly-half and side-stepped three defenders to go under the posts a come-back was on the cards.
However the timely entrance of North from the bench settled the game five minutes from time.
North ran through Cuthbert's attempted tackle before off-loading to lock Day and the second row charged over.
|Cardiff Blues Score Card|
|Scarlets Score Card|