How can airlines deny boarding based on blacklists despite issuing visas?

Travelers may be denied boarding by airlines based on various factors, including matching their names against a government blacklist or concerns regarding the legitimacy of their visas. Although embassies issue visas, airlines also conduct their own checks before departure, including verifying that passengers have onward flights. If a traveler's name appears on a list that indicates potential issues, or if airlines suspect that a passenger may not be allowed entry upon arrival, they have the authority to deny boarding. The process is primarily enforced through systems like the Advanced Passenger Processing System, which shares information between governments and airlines.