Why is the text message sent by Apple's mobile phone blue?

Blue indicates that outgoing messages are sent through iMessage.

IMessage information refers to text, photos or videos that you send to iOS devices and Macs through Wi-Fi or cellular mobile data networks. This information is usually encrypted and displayed in blue text bubbles. To turn iMessage information on or off, please go to Messages in Settings to modify it.

If you don't use iMessage information, you can use SMS/MMS. This information refers to the text and photos you send to other mobile phones or iOS devices. SMS/MMS is unencrypted and appears in a green text bubble on the device.

Extended data:

With iMessage, you can send and receive messages with any Mac user who uses iPad, iPhone, iPod touch or Mountain Lion, as well as send photos, videos, location information and contact information. If you have more than one Apple device, iMessage can keep an uninterrupted conversation on all devices.

In addition, you can send text messages, photos and videos to other mobile phones through the cellular network. You can even ask Siri to text you. Just say "Tell Xiaoli I'm on my way" and Siri will help you write and send messages.

References:

Information about iMessage and SMS/MMS -apple Support