In a medium saucepan, heat 2 tablespoons of olive oil over medium heat. Add the flour and whisk until combined, cooking for about 1 minute to create a roux. Gradually whisk in the vegetable stock, then add the chili powder, garlic powder, salt, ground cumin, and Mexican oregano. Bring the mixture to a simmer, stirring frequently until it thickens slightly.
In a skillet, heat 1 tablespoon of olive oil over medium heat. Add the diced onion and sauté until it becomes translucent, about 5 minutes.
In a mixing bowl, combine the sautéed onion, cooked black beans, chopped cilantro, and lime juice. Mix well to combine all the flavors.
To assemble the enchiladas, take a corn tortilla and spoon about 2-3 tablespoons of the black bean mixture into the center. Roll the tortilla tightly and place it seam-side down in the slow cooker. Repeat this process with all the tortillas.
Once all the tortillas are filled and arranged in the slow cooker, pour the prepared sauce over the top, ensuring all the enchiladas are well-coated. Sprinkle half of the shredded cheddar cheese on top.
Cover the slow cooker and cook on low for 4-6 hours or on high for 2-3 hours.
Once cooked, serve the enchiladas warm, garnished with the remaining cheddar cheese, a dollop of reduced-fat sour cream, and additional fresh cilantro.