Make the dressing. Stir together the mayo, Greek yogurt, hot sauce, garlic powder, salt, and black pepper in a large bowl.
Combine. Add the celery, carrot, and 3/4 of the scallion (reserving 1/4 for garnish), and toss to coat the veggies.
Finishing touches (right before serving). Gently fold in the blue cheese crumbles. Taste and add additional salt, black pepper, and hot sauce as desired. Add the remaining scallion on top.