Combine onion and 2 tablespoons vegetable broth in a pot over medium heat. Saute until onion becomes glassy, about 5 minutes. Add garlic and cumin and saute until fragrant, about 1 minute; avoid browning the garlic.
Add black beans, pumpkin puree, 1/2 cup vegetable broth, coconut milk, and chipotle pepper in adobo. Simmer until soup starts to thicken, about 15 minutes.
Stir in diced tomatoes and pumpkin pie spice. Coarsely blend with an immersion blender. Season with salt and pepper.