i am doing a christmas hamper for my parents, and to keep it cheap i am making some things myself. this is much quicker than other methods - no exact quantities, just use what you have and check the taste. sorry i have no pictures if the real thing, my camera is on the blink!
put your blackberries in a big pan, if they are frozen don't worry. turn them onto a low heat while you boil some water in the kettle.
when the water is boiled, almost cover the blackberries with the water, and turn up the heat. stir them, mashing as you go
about 2 minutes after adding the water, add some sugar (keep tasting it) until it tastes good, and continue mashing with your spoon. if this isn't going brilliantly or you're impatient, use a potato masher.
sift in some cornflour (sifting is important with corflour, trust me. and i mean some, about a tbsp at the most) and keep stirring. put the lid on and leave it on a low heat for as long as you've got. (half an hour is fine)
remove the lid to let any excess moisture evaparate. the jam should be quite thick when it has all evaparated. sprinke in a PINCH of gelatine and pour into a jar.
you're pretty much done now. leave it to set in the fridge, if you are a veggie and don't do gelatin, just add a little extra cornflour but make sure you boil it out or it will taste like flour! TIP: i got my jars from ikea Â£1.99 for 4.