The Newbridge advantage at the top of the table is cut to just three points and Blackwood's Lyn Hall believes the victory has opened up the division.
"This could be the catalyst for us to push on and to see what the season now holds for us" said Hall.
Leading the chase behind the top two are now Caerphilly, who capitalised on Merthyr's slip-up at Bargoed to take third place following their 22-12 victory over Ystrad Rhondda 22-12. Paul Adams and Stefan Sankala were try scorers for the Cheesemen.
Merthyr fell to fifth in the table after losing at Bargoed, who leapt above their visitors thanks to tries from Matthew Hutcheon and Geraint Hodges.
It took a last-minute try from Andrew Carlson for Rumney to break the deadlock in a tightly-fought 5-0 win over Newport Saracens, while the students of UWIC had no such problems scoring three tries in a 30-0 win at Beddau.
Carmarthen Quins had no trouble in keeping their 100 per cent record in Division One West when they hosted Dunvant.
Tries from Gavin Evans, Andrew Thomas, Alun Davies and two from Jamie Davies saw the visitors routed 27-0.
Quins picked up their sixth bonus point of the season, extending their lead at the top to 19 points.
With Tonmawr in Swalec Cup action, Narberth moved into second spot as captain Steve Martin scored two tries in their 26-15 win at Cwmllynfell.
Corus leapt above Carmarthen Athletic into fourth after beating them 34-15, Maesteg defeated Bonymaen 28-15 and Whitland saw off Felinfoel 18-12.