Combine the black beans, corn, tomatoes, red onion, cilantro, lime juice, salt, and cumin in a mixing bowl. Stir until the ingredients are evenly mixed.
Divide the salsa mixture among four small cups, filling each about three-quarters full.
Spoon the sour cream into a small zip-top bag and cut off one tiny corner to create a piping bag. Pipe a spiral of sour cream over the salsa in each cup.
Drag a toothpick from the center of the spiral outward in straight lines to form the web pattern on top of each cup.