Place 4 half pint (8 oz.) mason jars in a large stock pot/canning pot. Add enough water to cover the jars and bring to simmer.

Place the lids into a small saucepan and simmer those in a bit of water as well. No need to warm the rings, just the lids. (Make sure you are using new lids, it's best not to reuse them.)

Put a small plate in the freezer to test doneness for jam.

Add blueberries to a large pot and crush with potato masher. Add pectin to crushed blueberries and stir to combine.

Add sugar and lime juice to mixture, stir to combine and bring to a boil.

Bring jam to a rolling boil that cannot be stirred down. Boil for 8 - 10 minutes, stirring constantly.

Remove from heat and test jam for gel stage by putting a teaspoon of jam on the plate from the freezer. Place plate in freezer for one minute and swipe your finger through the jam. If it stays separated, jam is ready. If not, boil an additional minute and test again. (It's better to undercook than overcook.)

If jam is ready, add zest and stir to combine. If jam needs to boil an additional minute, add zest after you've boiled again.

Using canning tongs (or regular tongs with a dish towel wrapped around them) remove the jars from the water, pour the water out and set a wide mouth funnel inside of one jar.

Using a ladle or measuring cup, scoop jam and pour into warm jar leaving 1/4" of space at the top. Repeat until all 4 jars are filled, then wipe the rim of the jar with a damp towel, making sure the thread of the jar is clean.

Remove the lids from the simmering water, screw on the rings but do not tighten all the way. Enough to secure the lid, but not completely tightened. Repeat with remaining jars.

Place jars into a canning rack if you have one and lower into water in canning/stock pot. If not, use tongs and lower the jars one by one (fairly quickly) into the water. Make sure water covers the jars by at least two inches.

Bring water to a full boil. Boil for 10 - 12 minutes, then remove from water. (I always place my hot jars on top of a dishtowel on the counter so nothing slides around.)

Leave jars for 24 hours, you will hopefully hear that beautiful "PING" sound fairly soon after they've come out of the water, but it may take a little while.

To make sure the jar has sealed properly - press on the lid. If it has ANY give whatsoever, it has to go in the fridge. Otherwise, your jam is good to go! Remove rings, if desired. You don't need them anymore.