Combine the blueberries and water in a large saucepan and bring to a boil over medium-high heat. Boil for 5 minutes to break down the berries and release their juice. Using a potato masher or the back of a wooden spoon, crush the berries thoroughly against the side of the pan to extract as much liquid and flavor as possible. Continue cooking for another 5 minutes, stirring occasionally to ensure even breakdown.

Place a fine-mesh strainer over a bowl and carefully pour the blueberry mixture through it, allowing the liquid to drain completely. Press gently on the solids with the back of a spoon to extract every bit of juice, then discard the leftover pulp and seeds. You should have roughly 1 1/2 cups of clear blueberry juice.

Return the strained blueberry juice from Step 2 to the saucepan over medium heat. In a small bowl, whisk together the pectin and sugar until well combined—this prevents clumping when it hits the hot liquid. Slowly pour the pectin-sugar mixture into the juice while stirring constantly to ensure it dissolves smoothly and evenly throughout.

Add the lemon juice and salt to the pan and stir well. Increase the heat to medium-high and bring the mixture to a gentle boil, stirring frequently to prevent sticking. I like to add the lemon juice and salt at this stage rather than earlier because the acidity helps activate the pectin's setting power and the salt heightens the blueberry flavor without making it taste salty. Simmer for 8-12 minutes, stirring occasionally, until the mixture reaches setting point—it should coat the back of a spoon and hold its shape when a small amount is dropped on a cold plate.

Remove the pan from heat and let the jelly rest for 2-3 minutes to allow any foam to settle. Carefully pour the hot jelly into clean jars, leaving about 1/4 inch of space at the top. Allow the jelly to cool at room temperature for at least 30 minutes before covering—this helps the jelly set properly and gives you the best texture.