Put the blueberries in a large bowl with cold water. If you don't want any unripe (sour) blueberries in your cheong, pick out the ones that float. I like both sweet and sour blueberries, so I just leave them.
Add 1 tbsp of baking soda, gently mix, then let it soak for about 15 minutes. This is to wash the blueberries of any pesticides and/or dirt.
Rinse well then dry the blueberries either with a salad spinner or laying them out on paper towels.
There are two methods you can use here- the fast way is to muddle the blueberries before adding the sugar. You don't have to muddle every single blueberry into pulp, as you can see in this picture there are still a few whole blueberries. The more traditional way is to add the blueberries and sugar to the jar in layers. This will take a few weeks to start turning into syrup, and will have a slightly more concentrated flavor but honestly, it's not that noticeable to me compared to the fast way.*Note: make sure your jar is sanitized!
After the blueberries have been muddled, add about 2/3 of the sugar and mix well.
Pour in the remaining sugar on top, and evenly spread it to cover the blueberries. This is just an extra protective layer so mold doesn't form. It's going to completely dissolve eventually. Store in the cool dark spot for 1 week.If it's warm in your house, I recommend putting this in the refrigerator to prevent any fermentation from happening.
After 1 week, strain the syrup. Push/squeeze as much syrup as you can through. You can use what's remaining to make a blueberry soju so don't throw it away. Put it back in the jar and fill it with soju for a few weeks. Or you can use it to make something else, like jam.
Strain one more time into a sanitized glass jar or bottle.