How to make rhubarb and blackberry jam

I have never made rhubarb and blackberry jam before and I have never seen a recipe for it. I invented this one because I had some spare rhubarb, the blackberries are in season and I wanted to demonstrate the principles of jam making. Basically, you need pectin, sugar, acid and heat. That combination should mean your jam sets, subject to there being sufficient of each.

Anyway, this jam tasted rather pleasant.