Place berries in a large glass or stainless steel bowl. If berries are frozen, allow them to thaw somewhat before lightly mashing the fruit. The goal is not to puree the fruit, just lightly crush the raspberries and blackberries and “pop” as many of the blueberries as you can.
Add the remaining ingredients and fold together to moisten all the sugar. Allow mixture to rest 30 minutes to dissolve sugar.
After 30 minutes, stir the berry mixture until all sugar is dissolved. Try to catch any blueberries that are still whole and “pop” them on the side of the bowl with a spatula.
Let the mixture continue to rest for another 90 minutes, stirring every half hour or so. Yes, the 2 hour wait is necessary. During this time the natural pectins in the fruit are being drawn out – a process known as maceration. Without those pectins, your jam will never set.
While your berries are macerating, prepare your canning jars and a hot water bath for processing your filled jars.
Carefully pour the berry mixture into a large, heavy bottomed stainless steel or other non-reactive pot. Over medium heat, bring mixture to a fairly vigorous boil, stirring often. Any whole blueberries will rise to the top – just ease them over to the side of the pan and gently press on them with a spatula until they burst. Be careful not to splatter yourself!
Continue to boil the mixture, stirring frequently to prevent sticking/scorching until jam reaches the gelling point (this should take anywhere from 10-15 minutes). Technically the gelling point is approximately 8 degrees F above the temperature at which water boils. It’s helpful to take the temperature of a pot of boiling water to see what your target is (the higher the altitude you live at, the lower the temperature at which water will boil, and thus the lower the temp you will need to achieve to get your jam to gel).
You can either use an instant read thermometer to check when your jam reaches the gelling point. Or, you can tell that the jam has reached the gel point when it thickens noticeably. When you see the jam has thickened, turn off the heat and let the jam rest a minute. If the jam has successfully reached the gelling point, you will see a thin “crinkly” skin on the top of the jam when you gently drag your spatula thru the mixture.
If you don’t see that crinkly skin, bring the jam back to a boil and cook another minute or so. Keep testing till you see your jam is ready.
Pour cooked jam into sterilized jars, seal and process in a boiling water bath for 10 minutes. Turn the heat off the boiling water bath and let the jars rest in the water for 5 minutes. Remove jars from water and allow to come to room temp. While it’s still hot, the jam will appear loose. Don’t panic – as the jam cools to room temp it will set up. Make sure all jars are properly sealed before storing.