New problems using Black Masks

In trying to repeat my first exercise of adding skin texture variants to a character head. For example, forehead skin texture to forehead, cheek texture to cheek etc, etc (which worked exactly as the tutorial explains the first time I experimented 4 months ago) I now seeem to have odd results when I paint white onto the black mask. Instead of just revealing the texture in the localised area required, it also adds a shading effect around the outside of the stroke that creates the illusion of an indentation which you can see on the attachment. What am I doing wrong now that I was't doing when I first tried this 4 months ago? Any help would be greatly appreciated.