On the stovetop, mix together the blueberries, orange juice, water, sugar, and cinnamon. You may want to add a little more or less water depending on how thick you want the sauce to be. Half a cup seems to be just about perfect for us. Place the saucepan over a medium/low heat.

As the sauce is cooking, use some kind chop and mixing tool (like this one) to chop up the blueberries into the sauce. This will make a chunky sauce. If you want a smoother sauce you could use an immersion blender.

Let the mixture simmer on the stove for several minutes, stirring occasionally. In a small bowl or cup, blend the starch with just enough water to make a very thick liquid. Add the starch liquid into the sauce slowly, while stirring. It should thicken up immediately. After you've blended the starch and the sauce has thickened, you can either add in more water (if you feel like you'd like a thinner sauce), or just go ahead and remove from heat. I usually let the mixture sit for at least 30 minutes to an hour before serving just to let the flavors blend together. You can store the sauce in the fridge if you don't plan on using it right away. It's very tasty the next day!