An adjustment layer changes only the appearance of the layers below it without affecting those lower layers. For example, all color layers below a black-and-white adjustment layer will look black and white, and when I paint a red line on such a color layer it will appear as if I am drawing a gray line, but when I turn the black-and-white adjustment layer off, I will see the color layer with its red line. So I can work with colors, while seeing their greyscale values.
In the same way I would like to have a "blur layer", that will make everything below it appear blurred, while the actual layers remain crisp and sharp.
I would like to work on an image while seeing it blurred, but then be able to turn off the blur and have a sharp image.