Put 1 can of black beans in a blender with the chicken or vegetable stock, salsa and ground cumin. Blend until smooth. Put into a saucepan with the other can of beans and heat over medium heat. Taste and add salt and pepper if needed. Add the lime juice.
For Garnish:
Stir some lime juice into the sour cream until smooth and swirl some over the top of each serving and garnish with chopped green onions or cilantro.