How do messages sent between Apple phones turn blue?

You mean imessage, free texting between Apple phones?

Settings-Information -iMessage- Select On.

Then enter the normal SMS interface. If the other party has enabled iMessage, it will be automatically converted to iMessage, and the color is blue, which is different from the green of normal SMS.

Note that the device must be connected to the network when it is used, and the object of receiving information is also the person who has enabled iMessage and connected to the network.