"Blue planet" is a name that has been used since people entered outer space. Many astronauts in the world have left the earth by artificial satellites and spacecraft and went to distant space. The earth they see in space is like a round blue moon.
Why is the earth blue when viewed from space? This is because most parts of the earth are sea water, and the sea water shows blue under the irradiation of the sun. So people often call the earth "blue planet".