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.