Set the Instant Pot to saute. Add the oil and once shimmering, but not smoking, add the onion, mushrooms and cook, stirring, until the onions translucent and soft, and the mushrooms have started to take color, ~5 minutes.
Add the garlic, chili powder, cumin and oregano, chilies, and cook, stirring, until fragrant, about 1 minute. Add the beans, and stock. Set to pressure cook on high for 1 hour.
After the pressure cook cycle is complete, perform quick release. Then remove 1 cup of the beans with a slotted spoon and set aside.
Mash the beans in the pot with a potato masher or use an immersion blender to blend the black beans with 1 tablespoon salt until thick and creamy.
Stir in the reserved whole beans and season to taste with salt and pepper.
Use an immersion blender to blend the black beans with 1 tablespoon salt until thick and creamy. Stir in the reserved whole beans and season to taste with salt and pepper.