In a medium saucepan, toss the blackberries with the caster sugar. Stir to combine and let stand for one hour.
Squeeze the lemon over the blackberry sugar mixture, add the squeezed lemon to the saucepan and bring to a simmer over medium-high heat. Skim off any scum that has risen to the surface.
Add the balsamic vinegar and simmer, stirring, until the sugar is completely dissolved.
Reduce heat to medium and simmer, stirring occasionally and mashing blackberries, for about 30 minutes or until you’ve reached desired consistency.