Combine the cherries, simple syrup, balsamic vinegar, and water in the blender.
Blend on high speed until the mixture is completely smooth, and the cherries are fully broken down, about 30-45 seconds. You should see a vibrant red, slightly foamy mixture take shape.
Check the consistency—it should be thick and icy; if it's too runny, add a few more cherries and blend again. If it’s too thick, add a little more water and blend briefly.
Pour the slushie mixture into chilled glasses, filling them almost to the top with the icy blend.
Serve immediately with a straw or spoon to enjoy the icy, refreshing texture.