Changing backdrop filter on menu drop down

Hi everyone hope you’re well. Any help on the below would be much appreciated please?

My navbar has a transparent background, but when the menu button is clicked I’d like a backdrop blur filter to appear (so it matches the dropdown menu which has a backdrop blur filter). I can’t figure out how to have the transparent background animate to a have a backdrop filter blur?

I have attached an image to show what I mean.

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