Color blend mode producing grey

Hoping someone can explain...

I've been using a layer set to Color blend mode when trying to make some

minor skin color changes (no time to use frequency separation).

In most cases it works fine but on certain areas, choosing an obvious skin tone and painting it over another area is producing a totally desaturated gray color.

The color produced is far from the sampled color and I just can't make sense of it, since the blend color is not being painted over an area of low saturation.

I'm using a fully updated copy of Ps on windows

any advice?