I understand what the phrase means but It's a little bit awkward. I think '이탈리아 사람들은 까만색 고양이를 보면 복이 나간다고 생각합니다.' is better. first of all, '고양' is not right word. '고양이' is correct. secondly, 이탈리아 사람들'이' is grammatically correct but it could make me think that someone thinks luck disappears when Italian see a black cat. but It is just a little bit awkward but It can be written so.