3+ C blackberries
2 C sugar
lemon juice to taste (almost 1/3 C)
Preparation: Combine berries and sugar; refrigerate 24 hours, stirring occasionally. Macerating the fruit this way draws out the liquid out.
Just before cooking, add lemon (a little at a time, taste test as you go until the fruit flavor really 'jumps forward').
Cooking: Bring it all up to temp (boiling), dissolving all the sugar if it's not already. At first, there's lots of liquid and it's lighter in color. If foam develops, remove from heat briefly to skim off the foam. As it continues to cook, the volume will reduce; the color will darken; and the bubbles start to get smaller, shinier/glassier and form small 'wells'; and it begins to sound almost like a rustling.
If you want seedless jam (jelly), now is the time to pass the jam through a strainer; then complete the cooking process.
As it nears 'done-ness' it's thick enough that as you draw a spatula across the pan it will clearly leave a track before syrupy-thick, hot jam covers the bottom of the pot again. Many of the berries are broken down by this point and the jam is more cohesive, rather than chunks of berries in liquid.
The best test for 'done-ness' is to remove it from the heat, scoop out a small spoonful and put it in the freezer a few minutes -- just to bring it more quickly to room temperature -- to test the texture for a spreadable consistency.
Pour or spoon jam in to sterile jars; seal.