A simple no-pectin blackberry jam recipe infused with lemony Mexican mint marigold.
In a large, wide, non-reactive pot (a Dutch oven is perfect), stir to combine mashed blackberries, sugar, and lemon zest and juice. Tie up Mexican mint marigold leaves in a cheese cloth and immerse it in the jam. Bring mixture just to a boil, then turn heat down to medium or just hot enough that jam bubbles gently. Cook, stirring frequently, 15 - 20 minutes. Look for the jam bubbles to thicken and change sound, and for drips to thicken.
Check jam thickness by spooning a bit onto chilled spoon and pushing with fingertip. I found this jam was set when it wrinkled just a bit – it firms up considerably once cool.
Discard herbs in the cheesecloth.