Thanks for taking a look esion and vladimirjp - I am a bit confused though - I used the vertex normal edit tool to edit the normals before building the blend shapes and binding the head to the skeleton.

Does that mean that I have to edit the normals after binding the head to the skeleton?

The only difference between the two pictures above, is that I have rotated the neck joint and then these dark patches appeared. I thought that you had to edit the normals before binding geometry?

I am pretty sure that it doesn't have anything to do with painting weights.

Thanks again for taking a look.