In complete agreement with vbd. Some additional info:
1. Why is there a 'τε' between the adjective 'μέλαινά' and the noun 'Νὺξ'?
Note that τε is always postpositive, i.e., it always
follows the word it conjoins with another word.
What roles does the 'δ'' play?
δέ is the most common "connective" in Greek, and it's also postpositive (follows the first element in a sentence). You'll find it introducing at least half of all sentences. We don't need such connectives in English, since we show connection with a previous thought by simply raising or modulating our voice (Greek couldn't do that because of its system of tonal accentuation, where different vocal tones had semantic value).
Is the ' αὖτ' ' short for 'αὖταρ' or 'αὖτος' ?
Nope, you can't contract consonants, only vowels.
How do I know the subject of this clause is 'Νὺξ'?
Mιγεῖσα (literally, "having been mixed") is the feminine singular nominative aorist participle of μίγνυμι, "mix." As such, it must modify the only nominative singular feminine noun in the sentence, which is Νὺξ.