Wash and strain blackberries, then add them to a medium saucepan or Dutch oven, along with lemon juice, orange juice, and orange zest. Mix in sugar over medium-high heat until it dissolves.
Allow mixture to vigorously boil for 5 minutes, stirring often. Reduce to a simmer and add the sage leaves and a tea strainer or cheesecloth with the seeds of 1 lemon.
Remove the sage leaves after 3 minutes unless you want a stronger flavor. Allow the jam to simmer gently for 15-30 minutes or until blackberries begin to break down and jam starts to thicken. Use a potato masher to help the berries break down, if necessary.
Remove from the heat and discard the lemon seeds. You can test the jam's consistency by spooning a few tablespoons onto a saucer and refrigerating for 15 minutes. If it's not thick enough, boil the jam a few more minutes until the desired texture is achieved. Ladle the hot jam into sterilized jars, leaving ¼ inch headspace.
Notes
As the jam thickens, the bubbles will go from small and foamy to larger, glossier bubbles. If the bubbles look thick and lava-like, you're getting close. Jam will thicken even more once it cools and can burn if left to simmer for too long, so pay close attention.
For long-term storage, follow the canning instructions in this boysenberry jam recipe.
If you have some extra time, you can macerate the blackberries by letting them sit with the sugar for 20-30 minutes before following this blackberry jam recipe. This draws out juice, helps the sugar dissolve faster, and deepens the flavor.
Sage is delicate and can turn overpowering or disappear entirely if not handled properly. Overcooking sage can make it taste medicinal, while adding it too early can cause the flavor to fade during long cooking.
If the sage is too strong, simmering it a bit longer should help mellow the flavor.
Once it starts to thicken, stay close and stir frequently to prevent scorching.
When stored in a clean, airtight jar, this blackberry jam recipe will keep in the fridge for at least 2-3 weeks. If properly canned in a water bath, it can be stored at room temperature in the pantry for up to a year.
If your jam ends up thicker than you'd like, there's an easy fix. Gently reheat it, if necessary, then stir in 1-2 tablespoons of water at a time until it reaches the desired consistency.