With the Instant Pot on sauté mode, heat the oil until glistening. Add the onion and cook, stirring often, until translucent, about 3 minutes.
Add the black beans, stock, water, taco seasoning, and salt. Stir to combine, then secure the lid in place with the valve in the closed position. Turn the Instant Pot to manual mode with HIGH pressure selected for 25 minutes.
When the timer is done, allow the pressure to release naturally, about 20 minutes. If there is any pressure still in the pot, manually release it at this time.
Open the Instant Pot, stir, and let the beans rest for 5 minutes before serving. Store refrigerated in an airtight container for up to 1 week.