Reduplicated ABB pattern (黑 + 乎乎) is a productive descriptor template — same shape gives 白乎乎 'whitish', 胖乎乎 'chubby', 热乎乎 'piping hot'. The 乎乎 suffix softens an adjective and adds a tactile, vivid feel; 黑乎乎 is more colloquial than the plain 黑.
usage
Listed in CC-CEDICT as a variant of 黑糊糊. Both are accepted; 黑糊糊 may feel slightly more written.
