Just before serving time, peel, core, and slice the pears. Combine the greens, pears, walnuts, and blue cheese in a large salad bowl. Drizzle half the Port-Shallot Dressing over the top and toss. Drizzle in the remaining dressing, tossing until the greens are well coated-you may not need to add all the dressing. (Leftover dressing can be kept refrigerated for up to 1 week for another use.) Season with salt and pepper, and toss again.
Place on chilled salad plates and serve.
Pour the port into a small heavy saucepan over medium heat. Add the shallots and bring to a boil. Reduce the heat to low and cook for 15 minutes, or until the port is reduced to 2/3 cup without the shallots. Set aside to let cool.
Pour the port and shallots into the conainer of a blender and puree. Add the vinegar and blend well. While the motor is running, drizzle the oil through the hole in the blender top and continue to blend until emulsified and smooth. Season with salt and pepper, to taste. Set aside; refrigerate if not using immediately.