This is the 6531st most frequent Polish word.
czarna
"Czarna" means "black" (feminine form) in English.
Here, 'czarną' is the feminine singular accusative form of 'czarna', matching 'koszulę', which is feminine.
Mam czarną koszulę.
I have a black shirt.
In this sentence, 'czarne' is the neuter singular nominative form of 'czarny', matching 'niebo', which is neuter.
Niebo było czarne wieczorem.
The sky was black in the evening.
The adjective 'czarną' in this sentence is the feminine singular accusative form of 'czarna', agreeing with 'kotkę', which is feminine.