。
She was wearing black clothes.
Literal
She [topic-は] black clothes [object-を] was-wearing.
黒い is the everyday i-adjective form of 'black,' while 黒 alone (as in 黒の or 黒ずくめ) functions as a noun. Where 黒ずくめの服装 emphasizes 'all-black-from-head-to-toe' (with the suffix ~ずくめ doing the all-encompassing work), the bare 黒い服 just says 'black clothes' without that flavor. The choice between the two forms of the color word — 黒 vs 黒い — is a small stylistic dial Japanese uses constantly.