。
The tree looked black against the sky.
Literal
Tree [topic-は] sky [object-を] background-[using-にして] black-ly looked.
~を背景にして ('with X as a background / against the backdrop of X') is a stock formal phrase — literally 'making X the background.' Useful in descriptive writing. 黒く is the adverbial form of 黒い, and 見える ('is visible / looks') takes the resulting appearance as a manner adverbial. Compare 黒く見える ('looks black') with 黒い ('is black') — an important distinction in Japanese between 'seeming X' and 'being X.'