Having already beaten their regional neighbours this season, the Scarlets completed the double thanks to tries from Iolo Evans, Rheon James, Luke Treharne and George North with James Russell adding four conversions and as many penalties to secure victory.
Bowdy Davies, Ashton Evans and Scott Kelly crossed for the visitors at a wet and windy Drovers Field with Owen Williams kicking a conversion and penalty and Cameron Clement adding two conversions and a penalty to boot.
As Scarlets team manager Rob Evans explained, it was very much a story of two halves.
"For the first 40 minutes, the Ospreys were dominant and led 13-6 at the interval. They also scored first in the second half and at stage, we were obviously behind the black ball.
"However, I have to give the boys real credit for the manner in which they came back. Admittedly, the elements were in their favour, but to score 31 unanswered points against a side like the Ospreys is a terrific feat."
For the Blues, victory came with a bonus point too.
Having swept all before them so far this term, they ran out comfortable winners in the end, despite trailing 11-3 at one stage in the first half.
Owen Sheppeard, Jordan Sienawski, Alwyn Lee, Sam Campbell and Harry Robinson all touched down for tries with Joseff Griffin kicking a conversion and two penalties for good measure.
Leading scorer Matthew O'Brien crossed for the Dragons' only try and added three penalties, but the visitors were too strong and maintained their unbeaten run at the half way stage of their season.
Team manager Sophie Bennett said; "It was another competitive game which says a great deal about the competition. However, in the second half I thought we were excellent. To come here and gain a bonus point is a real boost."