b29driver wrote:
I am not familiar with the term "blending" a polished aircraft aluminum skin. The scratch has dented and stretched the skin. As a polished skin it cannot be repaired. ...
Polishing is, in essence, the same as "Blending", just on a smaller scale. To remove a deep scratch from any kind of material without leaving any (or much) evidence, an area much larger than the scratch will have to be worked. When the work is finished, the material's surface will transition smoothly down to the where the deepest point of the scratch was, and then back up again to the original surface level.
On polished skin, the blended area could polished again to match the surrounding area, but there's also considerations to be made about corrosion prevention and remaining thickness. The B-25 Structural Repair Manual may give patch repairs for something like this too.
Considering what a rare and special airplane a B-25 is these days, I agree with the suggestion that the owners may decide to go with a skin replacement.
This is all just speculation by a stranger on the internet, of course. Without knowing the dimensional details of the damage, and how important North American Aviation says that particular skin section is, an appropriate repair can't really be decided on.