Combine blackberries, sugar, lemon juice, and salt in a saucepan. Toss to coat and let sit for 5 minutes.

Place the saucepan over medium-high heat and bring to a boil. Reduce the heat to medium and simmer until sauce has reduced and thickened slightly, 15 to 20 minutes.

Remove from the heat and cool for a few minutes before serving, or refrigerate for later use; it will thicken further as it cools.