According to the Traditional English Pronunciation of Latin website, a syllable with secondary stress is by default pronounced with a short vowel: https://en.wikipedia.org/wiki/Traditional_English_pronunciation_of_Latin#Short_vowels (rule 4). So this would indicate that words with a nigro- prefix where ni- has secondary stress would be pronounced with a short i. Nigromantic is pronounced with a short i in the Oxford dictionary: https://www.oed.com/dictionary/nigromantic_adj, but nigrostriatal...
