Where does “মন্টেনেগ্ৰ'” come from?

মন্টেনেগ্ৰ' (Assamese) comes from English Montenegro, from Venetian Montenegro — black mountain.

Ancestry of “মন্টেনেগ্ৰ'”, step by step

Step	Language	Word	Meaning
1	English	Montenegro	—
2	Venetian	Montenegro	black mountain