The berries are ripest when they have turned from red to black. Since they ripen one by one, pick and store them in the refrigerator. Once you collect enough, you can make the jam.
I had a lot of small, firm blackberries, so I put them in the blender for 5 seconds and then strained them through a strainer. Even after doing that, they still had some seeds. If that bothers you, use a cheesecloth to strain again.
Place the strained berries into a pot. Add 40-50% the weight of the berries in sugar, and turn the heat to low. Bring it to a slow boil and occasionally remove any scum.
Stir the berries occasionally to prevent sticking. 1 second before it has become the consistency that you want, add the lemon juice and stop the heat.
Once cooled, place into a sterilized jar. This can keep in the refrigerator for up to 1 month.
Story Behind this Recipe
I wanted to pick the blackberries that grow in my backyard every year and eat them somehow, so I turned them into jam.
Don't blend too much and crush the seeds. The hard, little seeds can leave a taste in your mouth, so strain the berries as much as possible. I rarely see blackberries being sold in supermarkets here, but you can easily grow your own outside on the balcony. They have sharp thorns, but they will grow into such sweet berries.