In a medium pot over medium-high heat, add blackberries, sugar, water, and juice of 1 lemon.
Cook down the blackberries until they reduce down and start to burst, stirring often. Turn down the heat to medium-low and use the back of a wooden spoon or spatula to press the berries against the side of the pot to burst the berries.
Once the berries have reduced down and the mixture has thickened, about 30 minutes, remove pot from heat and let cool for 10 minutes.
Add the lemon essential oil and stir to incorporate.
Evenly divide jam mixture into mason jars and store in fridge for up to two months.
Notes
The fresh lemon juice will help thicken the mixture because of its natural pectin. If you don't like the consistency of this jam, you can add pectin powder to thicken up the jam some more.You'll want to add the lemon essential oil into the mixture when it's cooled because otherwise the heat will destroy the benefits of the oil. If you do not have lemon essential oil, I would use 1/2 teaspoon of lemon extract to start and adjust to taste!