Bring a large, deep skillet of water to a boil. Season with salt, add the carrots and cook until crisp-tender, 6 to 7 minutes. Using tongs, transfer the carrots to a large bowl of cold water; let cool. Drain; pat dry.
Meanwhile, place the coriander seeds in a resealable plastic freezer bag. Using a meat mallet or a rolling pin, crush the seeds. In a small bowl, combine the crushed seeds and the grapefruit juice. Gradually whisk in 3 tbsp. EVOO. Season the dressing with salt.
Preheat a grill to high. Brush the carrots with EVOO and grill, turning occasionally, until blistered, charred in spots and fork- tender, about 12 minutes. Transfer to a platter. Pour the grapefruit dressing over the carrots; turn to coat. Season with salt and garnish with the cilantro. Serve warm or at room temperature.