Place 1 can black beans and chicken broth into a blender. Cover and puree until smooth. Heat a large saucepan coated with cooking spray over medium-high heat; cook and stir onion and garlic until onion is tender, about 5 minutes. Stir remaining 1 can black beans and liquid, tomatoes, yogurt, lime juice, cumin, red pepper flakes, and pureed beans into onion mixture; bring to a boil. Reduce heat to low, cover, and simmer for 25 to 30 minutes, stirring occasionally. Garnish with cilantro to serve.