How do you make a filter affect more than just the boundaries of a PNG?

I am trying to add a filter to a transparent PNG but the filter (brush strokes) is constrained to only affect the inner parts of the PNG and not cross the boundaries.

PNG without filter

PNG with filter

I merged the PNG with the blue background (with spreads across the entire artboard) to see if that would allow the filter to affect the edges of the PNG, and it worked! (It might be a little hard to tell but the edges in this bottom picture expanded beyond the boundary, so the edges have more strokes)

The only problem is that I want the PNG to be separate from the background.

Any idea on how I can go about having the filter affect more than just the inner parts of the PNG?