The correct answer is: no article.
The word “blue” is an adjective that describes the shirt. Adjectives do not need articles.
The word “a” is an indefinite article. It is used to refer to a general or unspecified thing. In this case, the speaker is not referring to a specific blue shirt, but to any blue shirt.
The word “an” is an indefinite article. It is used to refer to a general or unspecified thing that begins with a vowel sound. In this case, the speaker is not referring to a specific blue shirt, but to any blue shirt that begins with a vowel sound.
The word “the” is a definite article. It is used to refer to a specific thing that has already been mentioned or is known to the listener. In this case, the speaker is referring to a specific blue shirt that has already been mentioned or is known to the listener.
Therefore, the correct answer is: no article.