Preheat grill to high. Rinse, and pat dry tilapia.
Spray one side of each tilapia piece with cooking spray. Sprinkle with desired amount of blackening seasoning. Flip and repeat.
If using a grill basket. spray generously with cooking spray. Note: If you don't have a grill basket, rub oil over grill grates before grilling fish.
Load tilapia into the grill basket. Place onto preheated grill. Grill 2 minutes, flip, and grill another 2 minutes or until fish is flaky.
In a large bowl, toss slaw with enough ranch dressing to coat but not get too runny.
To serve, place a half a piece of fish and desired amount of slaw into each tortilla.