The Collaborative International Dictionary of English v.0.48:

Darkish \Dark"ish\ (d[aum]r"k[i^]sh), a.
   Somewhat dark; dusky.
   [1913 Webster]

WordNet (r) 3.0 (2006):

darkish
    adj 1: slightly dark; "darkish red"

Moby Thesaurus II by Grady Ward, 1.0:

23 Moby Thesaurus words for "darkish":
   blackish, dark, dark-colored, darksome, dim, dimmish, dimpsy, dusk,
   dusky, funereal, grave, murk, murksome, murky, nigrescent, sad,
   semidark, sober, somber, sombrous, subfusc, swart, swarthy