Spray the inside of the bowl of your slow cooker with cooking spray. Add all ingredients to the bowl of the slow cooker, and stir until combined. (You can also stir the ingredients together in a separate bowl, and then transfer that to the bowl of the slow cooker afterwards.) Cook on high for 1.5 - 2 hours, or until the cheeses are all melted. (Alternately, you can also cook this on low for 3-4 hours, or until the cheeses are all melted.) Give the dip a good final stir, then transfer to a serving bowl and garnish with extra blue cheese and green onions if desired. Serve with chips or your desired dippers.