The victory sees Blues South go top of the table with 30 points but they will be looking nervously over their shoulders as second placed Ospreys are just one point behind, although they have played one game more.
Winger Joseph Williams went over in the first half for Blues South with fly half Callum Sheedy banging over the conversion.
In a tight encounter, full back James Whittingham crossed in the second half to edge his side clear after Matthew Paul had given Blues North hope with a try in the 66th minute.
Cory Sheppard had earlier closed the gap with a penalty for North after Tyler Williams had knocked over a penalty for South
North now find themselves in fifth place on the points table, just above RGC 1404.
* Monday night’s scheduled semi final of the Inter District competition between Blues North A and Blues South A was postponed and will now be played at a later date.
