Pour blackberry syrup over ice in a rocks glass, then add 2-3 dashes of bitters.
Pour in the bourbon and give it a gentle stir. Serve immediately.
Notes
If you don't have simple syrup, add a teaspoon of blackberry jam or muddle fresh blackberries in the glass.
Instead of using multiple small ice cubes, use a large ice cube, if you have one. It will melt slower, preventing dilution of the drink.
Adjust the sweetness by adding more or less blackberry simple syrup based on your preference. If you typically don't like sweet drinks, start with less and build up.