In a medium-sized saucepan, combine raspberries, blackberries, sugar, vinegar, and water.
Cook over medium heat, stirring occasionally, for 10 minutes.
Mix cornstarch with a little bit of water to form a paste and add it to the saucepan.
Continue cooking over medium heat for 5-7 minutes more, stirring occasionally until the sauce has thickened.
Let it cool & transfer the sauce to a blender & blend it.
Strain it through a sieve & discard the solids.
The sauce is ready to serve.