Preheat oven to 375°F and lightly grease a 9×13-inch baking dish.
In a bowl, mix black beans, crab, shrimp, 1/4 cup salsa, 1/2 cup cheese, and sour cream.
Spoon about 1/3 cup filling into each tortilla, roll tightly, and place seam-side down in the baking dish.
Melt butter in a skillet over medium heat. Whisk in flour until smooth.
Gradually add chicken broth, whisking constantly, until smooth and slightly thickened.
Stir in chili powder, cumin, salt, remaining salsa, 1/2 cup cheese, and half and half. Cook until cheese melts.
Pour sauce evenly over enchiladas. Cover with foil and bake for 30 minutes.
Remove foil, sprinkle remaining cheese on top, and bake 5–10 minutes until bubbly.
Serve over shredded cabbage and garnish with cilantro if desired.