Black refried beans with dried beans - Rinse dry beans and pour into a bowl with enough water to cover it + 3" above that. Soak overnight.
Next day drain that water and pour beans and sliced onion into a pot on the stove over medium high heat with enough water to cover them. Sprinkle in salt and allow to boil for about 1.5 hours or until super tender. Drain, set aside in a bowl.
Black refried beans with canned beans - Drain can and reserve liquids from the can if you want to use that to thin out instead of milk or broth.
Use a potato masher to smush softened beans as much as possible. Put a frying pan on stove over medium heat with oil and garlic for 2 minutes to fry. Add beans with all spices and mix together, warming for 2-3 minutes.
Add a few tbsp of milk, broth or water until it is the consistency you desire. Serve with feta on top.