Juice your lemons. You'll need 4-5 large lemons or 6-8 smaller lemons, probably; your mileage may vary.
Make a simple syrup by combining your cup of sugar and one cup of water in the sauce pan. Bring it to a simmer for 3-4 minutes to dissolve the sugar, then let it cool.
Add your blueberries to a blender or container and blend them until they're pureed.
Strain your blueberry mash to remove as much skin, seed, and pulp as possible while retaining as much liquid as you can. This will likely be slow and tedious; if you know a good way to do it, let me know in the comments!
Mix the simple syrup, lemon juice, and blueberry juice together, then add the remaining water and stir to combine. Add more water if you want a bit more, but thinner, lemonade.
Chill and serve or serve over ice right away.