Learn Farsi: در سیاهی شب هیچ‌چیز ندیدم. - I saw nothing in the blackness of the night.

It comes from the adjective سیاه (black). By adding the suffix ی (-i), we turn it into the abstract noun سیاهی (blackness or darkness). This is a very common way to make abstract nouns in Farsi, just like adding "-ness" in English.

How does the Ezafe work between سیاهی and شب?

To say "blackness of the night", we use an Ezafe. Because the abstract noun سیاهی ends in the vowel sound "i", the Ezafe takes the form of a "ye" sound to connect the words smoothly without pausing. So, the phrase is pronounced siyâhi-ye shab.

Why is the verb ندیدم (didn't see) negative if the sentence already says "nothing"?

Farsi requires a double negative. When you use words like هیچ‌چیز (nothing) or هیچ‌کس (nobody), the verb must also be negative. Literally, you are saying "I didn't see nothing". Here, the negative prefix ن is added to دیدم (I saw) to make ندیدم (I didn't see).

Could I use تاریکی instead of سیاهی in this sentence?

Yes, تاریکی means "darkness" (created by adding the same ی suffix to the adjective تاریک meaning dark) and is actually more common in everyday speech when talking about the night. Using سیاهی (blackness) gives the sentence a slightly more poetic or descriptive feel, emphasizing the pitch-black color.