To make spice blend, combine salt, cumin, oregano, paprika, sugar, and cayenne in a small bowl.
Heat 1 tablespoon oil in a large heavy skillet, such as cast iron, over medium heat. Add onion and cook until beginning to soften. Add 2 teaspoons spice blend to onions and continue to cook, stirring, until browning along edges, about 5 minutes. Transfer onions to a plate.
Return skillet to medium heat and add remaining tablespoon oil. Toss shrimp with remaining spice blend. Add shrimp to skillet and cook until blackened along edges and cooked through, about 5 minutes.
To make Cholula cream, combine sour cream, lime juice, hot sauce, and garlic in a small bowl. Season with salt.
Fill each tortilla with shrimp and onions. Top with Cholula Cream, cilantro, and lime zest.