Preheat the broiler. Microwave the squash on medium-high power until fork-tender, about 10 minutes; let cool slightly. Scoop out the flesh and transfer to a food processor; discard the skin.
Meanwhile, in a medium bowl, soak the red chiles in just enough boiling water to cover until softened, about 15 minutes; drain. Discard the stems and seeds. Add the red chiles and chili powder to the food processor; puree.
Place the poblano chiles on a baking sheet and broil, turning occasionally, until charred all over. Transfer to a bowl, cover and let sit until cool enough to handle, about 10 minutes. Discard the stems and seeds, then chop.
In a saucepan, heat the olive oil over medium-high heat. Add the onion and cook until softened, about 5 minutes. Stir in the squash puree, poblanos, black beans, oregano and 1 cup water. Bring to a simmer and cook for 15 minutes; season with salt and pepper. Stir in the sour cream.