adjective

of a colour intermediate between green and violet, as of the sky or sea on a sunny day.

the clear blue sky

sky blue,azure,cobalt (blue),sapphire,cerulean,navy (blue),saxe (blue),Oxford blue,Cambridge blue,ultramarine,lapis lazuli,indigo,aquamarine,turquoise,teal (blue),cyan,of the colour of the sky,of the colour of the sea

noun

blue colour or pigment.

she was dressed in blue

verb

make or become blue.

the light dims, bluing the retina