What will happen if it is blacklisted by the Singapore Immigration Bureau? Under what circumstances will it be blacklisted?

Something illegal. For example, illegal residence, illegal hukou, illegal employment, forged passport certificates, academic certificates, and engaging in abnormal occupations. Will be punished, recorded, warned, fined, imprisoned and flogged according to the seriousness of the case ... but regardless of the severity, these will be recorded. Small cases may be eliminated after 3 years, and serious cases will not be eliminated. Being blacklisted is usually a serious situation, and Singapore will refuse you entry. Even if you marry a Singaporean, you will still have visa problems. Visas in other countries have little impact. That's not necessarily true.