Combine the black beans with 3 quarts water in a Dutch oven or heavy stockpot over high heat.
Add the bay leaves, avocado leaves, onion, and garlic.
Bring to a boil, then lower to a simmer and cook, uncovered, until the beans are very tender, 2 to 3 hours.
While the beans are cooking, make the salt brine: combine 1 quart water with the salt and whisk until the salt is completely dissolved.
When the beans are tender, remove from the heat.
Pour them through a fine-mesh strainer, reserving all the cooking liquid and aromatics.
Add the beans to the brine and let them sit for at least 1 hour.
Drain the beans thoroughly, discarding the brine.
Combine the beans in a blender (preferably a high-powered one such as a Vitamix) with 1 cup of the cooking liquid and the bay leaves, avocado leaves, onion, and garlic from the pot.
Puree until smooth.
Add a little more cooking liquid if needed but only enough to help the blades turn.
Refrigerate in an airtight container for up to 1 week or freeze for up to 6 months.