In a large soup pot, heat the olive oil.
Sauté the onion and garlic in the olive oil for about 4-5 minutes.
Add the cumin and curry powder, and sauté for an additional 4-5 minutes.
Add the broth, thyme, pumpkin puree, salt, pepper and sugar. Bring to a boil.
Stir in the black beans and tomatoes.Reduce heat. Cover and gently simmer for 15 minutes, stirring occasionally.
Remove from heat and leave for a few minutes.Stir in the 1/4 cup nonfat sour cream and serve