Preheat oven to 350 degrees. Grease an 8 x 8 pan with cooking spray.In a medium bowl combine melted butter, flour, oats, sugar, cinnamon and salt. Stir until the mixture resembles wet sand and is clumpy.Set aside 1 cup of the mixture.
Transfer the remaining mixture into the prepared pan and pat down with your fingers or the backside of a spoon, set aside.
In a medium bowl combine blueberries, sugar, lemon juice and cornstarch. Toss to combine.
Layer the blueberry mixture over the crust.Sprinkle the remaining cup of crumb over the bars.
Bake for 30 minutes for fresh blueberries 50 minutes for frozen or until the center is set.
Allow the bars to cool completely on a wire rack before slicing.
I let them cool then place in the freezer for 15-30 minute to make it easier to cut. If you cut before they are cool it will make a mess.