Jordan Williams continued where he left off last week with another 17 points for the Scarlets. The outstanding outside half scored a try, three conversions and two penalties as the Scarlets picked up a second successive bonus point victory.
Hooker Thomas Bull, wing Jake Randall and openside flanker Daniel Thomas were also on the mark, whilst Blues South scored tries through Gareth Thompson and Jacob Morgan and Lloyd Chalk.
The Dragons scored the only tries of their game with the Ospreys at the Recreation Ground, but fell to a disappointing defeat.
Ethan Davies, who scored eight points in the victory over Blues North eight days ago, scored two tries as the Dragons led their visitors 12-6 at half time - scrum half Josh Guy kicking a conversion.
For the Ospreys, Ben Thomas kicked four penalties and Sam Davies dropped a goal to give the pre tournament favourites a welcome win.