Unless this is intentional?
My goal is occasionally to blur the contents of a layer while maintaining its edges by leaving "Preserve Transparency" enabled on that layer. The Smudge tool seems to respect this – I can smudge away without ever spilling coloured pixels beyond the content's original edges.
But the Blur tool actually fuzzes the edges, colouring pixels that were originally transparent and laid outside the layer's content.
Is something not working like it should? Let us know.