Cook 2 1/2 pounds small red potatoes in simmering water until just tender, about 20 minutes. Drain and pat dry, then toss with 2 tablespoons olive oil and 1 tablespoon kosher salt. Put the potatoes in a grilling basket (or on a wire rack set on the grill grates); grill over medium-high heat, turning often, until golden and crisp, about 10 minutes. Meanwhile, whisk 1/2 cup each sour cream and crumbled blue cheese with 1/3 cup buttermilk; season with salt and pepper. Transfer the potatoes to a bowl; top with the dressing, thinly sliced celery and carrot, celery leaves and more blue cheese.