Special equipment: Blender
Heat the oil in a medium saucepan over medium heat. Add the garlic, onions, jalapenos and 1/2 teaspoon salt and cook, stirring occasionally, until softened, about 6 minutes. Add the cumin and coriander and cook, stirring, until fragrant, about 1 minute. Add the chicken broth and black beans with their liquid. Increase the heat to medium high and bring to a boil. Reduce the heat to low and simmer, covered, for 10 minutes. Transfer about three-quarters of the soup to a blender and blend until smooth (be careful when blending hot liquids). Return the soup to the saucepan and season with salt. Serve topped with cilantro and sour cream.
When blending hot liquid, first let it cool for five minutes or so, then transfer it to a blender, filling only halfway. Put the lid on, leaving one corner open. Cover the lid with a kitchen towel to catch splatters, and pulse until smooth.