in a heavy 6 to 8 quart saucepan pour blueberry and lemon juice. Add grapes bring to a boil and simmer 20 minutes. I let this sit and steep as long as I can. I like to let it sit in the refrigerator over night.
Remove grapes. I like to use a strainer set over a bowl and squeeze the juice in the grapes. Add back to saucepan. Sprinkle the pectin over the juice and let sit 1 to 2 minutes to dissolve.
Bring to a full rolling boil over medium-high heat stirring frequently. Stir in sugar and return to a full boil for one minute, stirring constantly. Remove from heat and quickly skim off as much as you can.
Pour into hot sterilized jars, leaving 1/4 inches space. Process jars for 5 minutes in full boiling water.