Opposite number Ashley Smith levelled matters on 14 minutes, but Murphy's second, a minute later, restored the Blues' three point lead and that was how it remained until a five metre scrum, on 22 minutes, provided the Dragons with a perfect platform to score the opening try of the night. No.8 Lewis Evans picked up and Daniel Robinson arrived on the angle from inside centre to force his way over.
The Dragons posed a significant threat from the base of the scrum with Evans and scrum half Rob Lewis defying the conditions to keep their side on the front foot. In fact had it not have been for some understandable handling mistakes elsewhere, they might have been well clear at half time. As it was, the home side turned around with a modest two point lead and with only 14 men after lock Bryn Jenkins was yellow-carded for pulling down a rolling maul.
Murphy missed kickable penalties either side of the break as the Blues threatened to expose the absence of Jenkins and then hooked a drop goal wide of the posts. A second yellow for the Dragons, this time for Will Jones, meant another 10 minutes without a full complement during which time the Blues were awarded a penalty try by referee Ken Bowen. Rubbing salt into the wound Mr Bowen then sent Ben Flower to the bin.
The Dragons made light of their numerical problems after 55 minutes when a length of the field handling movement involving no fewer than nine players ended with replacement Chris Vitalini crossing. Blues captain and blindside Bradley Davies became the fourth player to be binned - for obstruction - and a minute after his departure, Lewis picked up, combined with Vitalini to send in left wing Marcus Johnstone for a try that won the day.
It was always likely to be a nip and tuck affair at Stradey Park between last season's finalists. However, the Scarlets made full use of home advantage on a filthy night to kick off their season on the right note. Darren Daniels, who has already progressed into the senior Scarlets side this season, crossed for a first half try which Rhys Priestland converted but a try from scrum half Martin roberts kept the Ospreys within two points at half time.
The second half was a different story altogether and tries from Steve Martin and a penalty try plus another 10 points from the boot of Priestland was enough to give the Scarlets the points. The Ospreys replied with a two tries from replacement scrum half Lee Gunnell and a penalty for,Jonathan Spratt, but it was too little too late in a game that saw Scarlets replacement hooker Ken Owens and Ospreys' Daniel Lloyd-Jones both yellow carded.
On Saturday, the under 16s take centre stage with the Dragons and Ospreys playing each other at Newbridge (11am) and the Blues North taking on Blues South at Brecon (11am). On Sunday it's the turn of the under 18s - the Blues entertain the Ospreys at Pontypridd's Sardis Road (12noon) whilst the Dragons andScarlets will meet at Caerphilly's Virginia Park (12noon).
Tries: Robinson, Vitalini, Johnstone
Try: Penalty try
Penalties: Murphy (2)
Tries: Daniels, Martin, penalty try
Conversions: Priestland (3)
Penalties: Priestland (2)
Tries: Roberts, Gunnell (2)