Prepare the grill for direct cooking at 500°F.
Whisk together the walnut oil, vinegar and honey to form an emulsion. Finely crumble 1/2 of the blue cheese into the emulsion and whisk vigorously to break down the cheese into the dressing. Reserve.
Lightly brush the two cut sides of each radicchio quarter with olive oil. Sprinkle with salt.
Grill the radicchio over direct heat for about 2 minutes on each cut side, about 4 minutes total. Remove from the grill, Drizzle generously with dressing. Crumble the remaining blue cheese over the top and sprinkle with bacon. Serve warm.