Coat a grill rack with oil and preheat the grill on high for 5 minutes.
Place the blackening seasoning in a shallow baking dish. Roll the catfish in the seasoning until well coated. Reduce the grill to medium-high, and grill the onions over direct heat until well browned and wilting, turning once, about 10 minutes. Meanwhile, grill the fish over direct heat until cooked through, turning once, about 6 minutes. At the same time, place the tortillas directly on the grill for 1 minute per side. Stack the tortillas and wrap them in foil or a clean towel to keep warm.
Arrange the fish on a plate, and flake into bite-size pieces with a fork. Transfer the onions to a separate plate. Place the tomatoes, avocado, sour cream, lime wedges and cilantro in separate bowls. Set out with the warm tortillas and hot sauce, and invite guests to make their own tacos.