In a small bowl, mix the smoked paprika, garlic powder, onion powder, cumin, cayenne if using, salt, and black pepper.
Pat the tilapia dry and brush both sides with olive oil. Rub the spice mixture evenly over each fillet.
Heat a large nonstick skillet over medium-high heat and add a small amount of olive oil if needed. Cook the fillets for 3 to 4 minutes per side until opaque and flaky. Remove from heat and let rest briefly before breaking into chunks.
Add the avocado, sour cream or Greek yogurt, lime juice, garlic, cilantro, olive oil, and salt to a blender or food processor. Blend until smooth. Add water a tablespoon at a time until the sauce reaches your preferred consistency.
Place shredded cabbage onto each warmed tortilla. Add chunks of blackened tilapia, drizzle with avocado cilantro sauce, and top with diced onion and chopped cilantro.
Serve with lime wedges and optional jalapeno slices.