'Possible fifteenth Black Painting' you're allowed to just add it to zhe Black Paintings because zhe collection called zhe 'Black Paintings' was a post-facto designation anyway as Goya himself never called zhem a collection and zhey were not fully discovered until after his death. Zhe collection called zhe 'Black Paintings' is a synzhetic creation, you can add or subtract individual pieces from it as much as you like
Stayed tuned for more 'Notes from zhe Art World' wizh Ada