The on-again/off-again nature of the leak is probably to do with how much time the camera (and the affected frame) has spent with the light leak facing the sun or some other bright light.
It is odd, though, that the leak shows up with a bluish tint. As was mentioned light leaks from the backside of color negative film are usually orange.
As the Leica winds the film on the take-up with the emulsion side out the leak is probably in the upper right corner where the back slides into the light trap channels. This would allow the leaked light direct access to the emulsion. Just a guess, mind.
As mentioned, try sealing around the back with black tape. In bright sun, with the lens cap on, give the leak plenty of exposure to the sun while advancing the film and peeling back the tape. The first frame showing leaked light, and the position of the pulled back tape when the camera was exposed to the sun, should narrow down the location of the leak.
Advice worth price paid.