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.
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.