You just want it to cool a bit before layering it into the crumble bars. Add the oats ... Set aside about 1 cup worth of the mixture for topping. Place the remainder into the prepared tin.

The blueberry mixture can be prepared and refrigerated at least 40 minutes and up to 3 days in advance. Store lightly covered at room temperature for up to 3 days, or freeze for up to 2 months.