Place potatoes in large pot. Cover with cold water; add salt. Boil until tender. Drain. Cool slightly. Peel Cut into 1-inch pieces. Transfer to large bowl.
Add mayo, vinegar, lemon juice and 1 lb of blue cheese into blender. Blend until liquefied. Add a splash of water if dressing is too thick. Do not add more than 1/4 cup of water.
Pan fry bacon until extra crispy. Allow bacon to drain and cool on paper towels. Rough chop into chunky pieces.
Cut green onions in half, length-wise. Finely chop white and green parts (you want them to be super thin and small).
Add bacon, green onions and remaining blue cheese crumbles to bowl with diced potatoes.
Pour blue cheese dressing over ingredients. Gently stir with rubber spatula, making sure to coat all ingredients. Cover with plastic wrap and refrigerate for 24 hours. The potatoes will have absorbed all the dressing and will seem a little “dry”, but they are full of blue cheesy goodness. Add a dollop of mayonnaise and combine just before serving.