Here let me give you a bit on pressing and knocking so you will see why a surface stain doesn't really matter and helps performance, in my view
[We know a stain is stiffer and stronger]
When a bat is pressed it creates a stiff surface layer on the face of the bat. If you press it to hard the willow fibre's combine together to deeply into the wood and the blade has no real rebound [ping] If it is to soft then lots and lots gentle knocking is required to stiffen up the blade and then more rigourous knocking. At the end of the day we all hope that we don't get an over pressed bat and then the basic idea is stiffen the surface
On the left side is surface stiffness along the bottom is hours spent knocking in.
Look at the dot shape and 0hrs of knocking in and then at 4 hours. The surface stiffness has doubled!!!!
So what has happened - The core of the bat is left unhardened resulting in a more elastic response of the bat.
Because the surface is stiffer less enery will be absorbed by the bat and so therefore it will be transfered to the ball
A blemish actually helps!!!!!
Also the reason why some bats play-in quickly, some take an age and some never get going at all!!