Instrumantal is used in sentences type "noun IS noun", i.e. that something is a member of some type of objects, like f.ex. "Pies jest zwierzęciem" [A dog is an animal].
Here you have a different statement: "something exists and has a property", it is a sample of very short sentence type "verb adjective noun" which serves to describe a state or circumstances - and translates into English as simple as to "pronoun verb adjective noun" - A pronoun is obligatory in English, but it is not in Polish.
- Jest późny wieczór (nominative case) [It is late in the evening]
- Jest wczesny ranek (nom.case) [It is early morning]
- Jest ładna pogoda [It is a nice weather]
Attention, it is something different than sentence type "noun IS adjective"! In the contrary, that structure serves to attribute a property to a noun:
- Wieczór (nominative case) jest późny [The evening is late (already)]
- Dziewczyna jest ładna [The girl is pretty]
- Jabłko jest czerwone [The apple is red]
noc is feminine. It is kind of exception as there are not many -c ending feminine words, but there are many feminine nouns that end with "softer" consonants (s,sz, ś, c,ć, cz ż ź dż dź ń), I always say that you should toss the coin or check dictionary for them. (ść ending is more likely to be feminine noun)
Yes, you could.
This is a nice sentence teaching an interesting nuance. Yes, you can understand the English sentence simply as "[It/This/That] is a dark night", constructed identically as let's say "[It/This/That] is a small dog", and then you translate it as "To [jest/] ciemna noc".
However, the main Polish sentence is constructed differently. "Jest ciemna noc" means that right now it is a dark night. It's similar in construction to "It is raining" ("Pada deszcz"), a sentence which (at least in Polish) doesn't have any grammatical subject, no real 'it' to translate - not even a dummy pronoun.
"Jest ciemna noc" is a subjectless sentence which means that right now we have a dark night. Sentences like this are used when English uses "It" and cannot substitute it with "This" or "That".
"To ciemna noc" is a correct answer, but it means "[This/That/It] is a dark night" (there is some subject, you say this 'about something').
Both interpretations (subjectless/with subject) are correct without further context.