attached is a screen shot

But not only has that left you with a jagged edge (rather than an anti-aliased black-to-white edge), it's still left you with some border pixels with a blue tinge that doesn't belong to them. What Dignan wants is a fill that knows the background and foreground colours, and replaces any adjacent pixel that's any linear combination of the two, with the same linear combination of foreground and new background.

This is technically feasible, but I don't know of a package that does it.

Peter