Preheat the oven to 375 degrees F (190 degrees C). Place the potatoes in a large saucepan with water to cover. Bring to a boil over medium-high heat, and cook until tender, about 8 to 10 minutes. Drain, and transfer to a casserole dish.
Melt the butter in a medium saucepan over medium-high heat. Whisk in the flour, and cook for 5 minutes, stirring constantly. Gradually whisk in the milk and cream so there are no lumps. Reduce heat and simmer for 20 minutes. Remove from heat and whisk in the blue cheese until smooth. Pour over the potatoes in the dish. Sprinkle breadcrumbs over the top.
Bake for 25 minutes in the preheated oven, or until top is nicely browned.