The Collaborative International Dictionary of English v.0.48:

Swarthiness \Swarth"i*ness\, n.
   The quality or state of being swarthy; a dusky or dark
   complexion; tawniness.
   [1913 Webster]

WordNet (r) 3.0 (2006):

swarthiness
    n 1: a swarthy complexion [syn: darkness, duskiness,
         swarthiness]

Moby Thesaurus II by Grady Ward, 1.0:

36 Moby Thesaurus words for "swarthiness":
   Egyptian darkness, Erebus, blackishness, blackness, dark,
   darkishness, darkness, darkness visible, darksomeness,
   dead of night, duskiness, duskness, funereality, graveness,
   intense darkness, lightlessness, moonlessness, night, obscure,
   obscure darkness, obscurity, pitch-darkness, pitchy darkness,
   sadness, soberness, sobriety, somberness, starlessness,
   sunlessness, swarth, swartness, tenebrosity, tenebrousness,
   the palpable obscure, total darkness, velvet darkness