Put your 5 empty glass jars into a large pot of water and bring it to a gentle simmer. Leaving them in hot water keeps the glass warm so it doesn't crack when you add hot jam later.
Boil the Fruit:
In a separate cooking pot, mix the peaches, blueberries, lemon juice, and pectin powder. Stirring constantly on high heat, bring it to a bubbling boil.
Add Sugar & Hard Boil:
Pour all 5 cups of sugar into the fruit pot at once. Stir well. Let the mixture come to an aggressive, violent boil that you can't stir away. Boil it hard for exactly 1 minute, then turn off the stove.
Fill and Seal:
Take your hot jars out of the water. Pour the hot jam inside, leaving 1/4 inch of empty space at the top. Wipe the glass rims clean with a damp paper towel, put the flat lids on, and screw the metal rings on until snug.
Boil to Seal:
Put your filled, closed jars back into the big pot of boiling water for 10 minutes to lock in the freshness. Remove them and let them cool on a towel for 24 hours.