Henrietta Howard, Countess of Suffolk, in masquerade dress by Thomas Gibson, 1720.
La Comtesse de Bersac, by Jean-Baptiste Santerre.
Called visor or vizzard, moretta or servetta muta, the little black mask was worn not only to masquerades, but also as an everyday accessory, to conceal one's identity or protect against the sun. The masks were held in place by pinching a bead between the teeth, which undoubtedly made masked conversation rather difficult.
It's not much different, I suppose, from the "facekini" apparently popular in China these days.