Put the berries, sugar and lemon juice in a small-medium saucepan and put on a medium heat. Stir as it heats to dissolve the sugar. Break up the berries with a spatula/spoon a little as they soften. As it cooks, prepare your jar/jars - either warm in a low over a few minutes or submerge in boiling water to sterilize. Take care using tongs/gloves to handle as you remove.

Bring the jam to a simmer, reduce the heat a little and simmer for approximately 15 minutes (or longer, as needed), stirring occasionally. The blackberries will initially become brighter then the mixture will become darker and more even in color. It will also start to thicken - when you stir a spoon/spatula through, it will take a bit longer to close in behind.

Remove the pan from the heat, allow to cool a minute or two then skim off any foam. Transfer to your prepared clean jar(s) and put lid on while still hot. Leave to cool.