I think I have a good way: Go to /System/Library/CoreServices/CoreTypes.bundle/
right click: Show package content
Contents/Resources/
there you will find a
com.apple.macbook-black.icns
and a
com.apple.macbook-white.icns
then change the "black" to "white" and safe the white before with another name.
from now on it should always display this picture of a Blackbook when it normally would display a white one.
I hope this works, I haven't tested it yet.
EDIT: yes it does work. When you want to rename it, it won't let you. So just drag the black one onto your desktop. Then rename it to white. And drag the white one also and save it to another name, so that you have a backup if you ever want to change it to original. Then you have to drag it back. It will ask you to identify (because you are altering system files). Click on it and enter your admin password. Then click "replace". Now you only have to restart, and you will see your Black Macbook Icon.
(aren't you having a Black MB? Why is your icon white in the first place?)