very very dark leaves is typically an indication of too much Nitrogen. however most people usually get nute burn first.
some strains have very dark leaves naturally... my grape ape had very dark almost black leaves on the mother from seed. her offspring however havent been quite as dark tho. it turned out to be a 100% purple budded phenotype.
go look up the pic for Rare Darkness. its grape ape x rd#1 and it shows what im talking about.