The Ospreys, made the short trip to Abercynon to play a rejuvenated North side and romped home 30-5 thanks to a hat trick of tries from outstanding centre Christian Phillips, a fourth from fellow centre Ashley Beck and a fifth from full back Scott Hancock.
Daniel Lewis scored the Blues' only points courtesy of a first half try.
In the west, however, there was much better news for the region with the South defeating the struggling Scarlets 21-15. Outside half Joe Deacon kicked six points with flanker Cory Hearn, scrum half Tom Marley and centre Rhys Meredith completing a fine night's work with tries.
A penalty try and a try from scrum half Tom James kept the Scarlets in touch but the Blues held on for a crucial away win.
The victory keeps the South in third place but the Ospreys leapfrog the Dragons to assume top spot despite having played a game more.