Add all the spices for the blackening spice mix to a small bowl, stir well, then set aside for now.
Chop your onions, celery, and bell pepper, and add to a large mixing bowl. Set aside for now.
Halve or quarter the potatoes (depending on your preferred size) and place them in a pot. Fill with water until all potatoes are submerged. Bring to a boil over medium heat on the stove and cook for about 10-12 minutes, or until tender.
Meanwhile, as potatoes are cooking: Toss your shrimp in the olive oil, then with 1 tbsp of the blackening spice mixture you prepared earlier.
Heat 1 tbsp butter in a skillet over medium heat. Add the shrimp and cook for about 2 minutes. Flip and cook for an additional 2-3 minutes, or until shrimp is cooked through. Remove from heat.
When potatoes are done, drain and add to your bowl with the chopped veggies. Add the shrimp.
In a small mixing bowl, whisk together the Greek yogurt, spicy mustard, lemon juice, and about 1 tbsp (more or less, according to taste) of the remaining blackening spice mix.
Pour the yogurt-based dressing over the potato/veggie/shrimp mixture. Toss everything well to combine. Serve warm and enjoy!