非法 is the general adjective for 'illegal, unlawful,' used in both formal and everyday language (e.g., 非法活动). 违法 specifically emphasizes 'violating the law' and is more formal, often used as a verb or in legal contexts (e.g., 违法行为). 不法 is a literary, older term found in fixed compounds (e.g., 不法分子), while 黑 is a colloquial prefix in words like 黑市.