Pat fish dry with paper towels, then cut fish filets in half lengthwise.
Brush both sides of fish with butter, then sprinkle all sides of the fish liberally with blackening seasoning, gently press into the fish.
Spray air fryer basket with cooking spray. Place fish in air fryer, do not overlap or crowd basket.
Air fry at 400°F for 5-6 minutes. Fish is done when internal temperature reaches 140°F. Remove fish from air fryer with spatula or tongs.
Warm tortillas, covered with a damp paper towel, in microwave 15-20 seconds.
To assemble tacos, top each tortilla with 1 piece of fish, slaw, a drizzle of sriracha mayonnaise and a squeeze of key lime.