Dynamic Blur Effect

Hello,

I know how to create a on mouse over blur effect - just have two images, one blured and one not, and use alpha to tween them it - but i have a site where the background does change, and i would like a dynamic bluring butten, the butten is pure black with alpha 20 is it is transparent, but when the user puts there mouse over the butten i would like it to blur.

I allready have tried inserting an image from photoshop that just has a blur applied but it seems that the blur effect will not blur the background behind, the blur effect is contained with in the image itself… any ideas ?

thanks.