blue
The Italian word for 'blue' is 'blu'. It is used in the same way as in English, to describe the color of something. For example, 'Il cielo è blu' means 'The sky is blue'.
The sea is blue.
This sentence is a basic statement describing the color of the sea. It is a typical use of the adjective 'blu', which is identical in English and Italian spelling, but different in pronunciation. This phrase can be used in any context where the color of the sea comes up in conversation.
The blue sky is clear.
This sentence expresses the observation that a blue sky is clear, suggesting good weather. The word 'blu' is used here as an adjective describing the sky. Just like in English, colors in Italian precede the noun they modify.
I bought a blue car.
This sentence tells that someone has bought a blue car. 'Blu', in this case, is again used as an adjective directly preceding the noun it modifies. Note that in Italian, unlike English, adjectives usually follow the noun they refer to, but color adjectives like 'blu' can precede or follow the noun with little to no change in meaning.