As the adjective is for Paphiopedilum it's: atratum (only single t'es!).
I have now consulted the litterature - and Braem, too, deals with the matter under P. villosum var. boxallii - but he cites Master's description of 'Cypripedium boxallii var. atratum':
" ... [it] is based on a "dwarf plant" with "dark green foliage not exceeding 8 in. [20 cm] in height, the flower stems topping those by almost 2 in. [5 cm]"
.... "The colours of the lip and lateral petals consist of reddish-purple, associated with light green; the dorsal sepal is green, black-brown speckled. The upper margin being white. The whole flower has the waxed appearance siin in C. villosum."" (Braem et al.: "The genus Paphiopedilum", 2nd edition. Dehra Dun, India, 2016)
The picture of your flower, alone, seems to correspond pretty well with Masters description - how about the characteristics of the rest of the plant?
Kind regards,
Jens