Texture painting - only white value shows on glass material

Hi ! Doing a little bit of texture painting for the first time and I have an issue here, only the white value of the texture painting mask is showing the material. There is no in-between with the grey scale, showing the material with less opacity. But it only do this on a glass material, and it works fine on a basic white material (see screeshots)

Do you know how to fix that ?

(the blue frame is the black texture in the shading editor screenshot)

Thanks!