Add the water, blueberries (or any fruit) and date to a blender and blend until smooth.
Serve immediately with some ice cubes if you like your agua fresca cold. Best when fresh, keep leftovers in a sealed container in the fridge for up to 2 days.
Notes
This drink can be made with any fruit you have on hand. I always add the same amount of water and fruit (1 cup each). One cup is always 250 ml of water, but every fruit has a different weight, so if you're not going to add blueberries, use a cup, don't add 250 g.
Add more or less water, depending on how thick you want your agua fresca.
Feel free to use fresh or frozen fruit.
If the fruit is sweet enough for you, don't add the date. If it's not, add more dates if needed. Any other sweetener is also okay.