Why is the first sentence of the new Apple mobile phone blue?

A specific keyword or expression was entered. In a new Apple mobile phone (such as iPhone), when using iMessage (SMS and MMS service of Apple mobile phone) to send messages, enter specific keywords or facial words, and these keywords or facial words will appear in blue. This blue display is a special function of iMessage. The blue screen function is mainly applicable to iMessage service, and both parties must use Apple devices and turn on iMessage to use it normally.