Using a sharp paring knife, peel three or four strips of lemon peel from the lemon. Skin should be about 1/2 inch wide and not have too much of the bitter pith or white part on it. Then juice the lemon, and set both zest and juice aside.
Place blueberries and water in a medium pot. Don’t worry about stems or leaves; they will be strained out later. Using a potato masher, crush the berries.
Over medium-high heat, bring the berries and water to a boil, then lower the temperature to medium-low. Simmer berries for 15 minutes, stirring occasionally. They will darken considerably.
Remove pot from heat and ladle berries into a fine sieve set over a heat-proof bowl or measuring cup. Using the back of a smaller ladle, press on the berry solids to extract as much juice as possible. Discard solids. (Update: or save them if you like. This part is apparently alarming to quite a few commenters! But my solids are full of stems, not worthy of a smoothie.)
Return the blueberry juices to the pot, then add the lemon peel (not juice yet) and the sugar. Bring to a boil and simmer for about 10 minutes until the mixture thickens slightly. (Optional: Add a stick of cinnamon here, if you like).
Add 2 tablespoons lemon juice and stir to combine. Boil another minute or two. Remove from heat and allow to cool. Remove lemon zest.
Using a funnel, pour syrup into clean jars. Top with lid and store in the refrigerator for up to three months, or the freezer for up to nine.
Recipe Update!A lot of you have asked about canning this syrup. Yes, I have done so with success. Brush up on your canning basics before you proceed, then ladle hot syrup into hot, sterilized jars (I use 1/4 pint), wipe rings, and cover with lids. Place in a hot water canning bath and process for 10 minutes.