Body part that might be blue

IRIS

["This colored ring around the pupil controls how much light enters your eye and can sometimes appear a different shade in certain lighting."]

This clue plays on the double meaning of "blue" to hint at the iris. In anatomy, the iris is the colored part of the eye, which can sometimes appear blue. The word "blue" also subtly nods to the phrase "feeling blue," but here it’s purely about the eye’s pigment—making "iris" the straightforward answer.