black as thunder vs dusky

black as thunder

• Very black (especially of a mood, a frown, etc.). 

dusky

• Ashen; having a greyish skin coloration. 

• Dimly lit, as at dusk (evening). 

• Having a shade of color that is rather dark. 

• A dusky dolphin. 

• A dusky grouse. 

• A dusky shark.