The title has at least two meanings, referring both to Pecola's desire to change the way she is seen and the way sees.
Let's deal with the easy one first. As a black child growing up in1940s America, Pecola associates beauty with being white and having blue eyes, like child icon Shirley Temple. Pecola seems to be OK with her nose and mouth, even her hair – but her eyes, oh, her eyes! She thinks that if she could just have those bright blue eyes, she'd become truly beautiful and no one would ever tease her at school, her parents wouldn't fight anymore, and she'd never be sad again.
Now, onto the second aspect of the title – Pecola's desire to see the world differently. Pecola believes that if her eyes were blue, she would begin to see the world the way that white children do – she would get to be innocent, she would experience a loving family.
A third idea plays with the meaning of "blue" as "sad." Pecola's eyes already are the bluest in the book, in that they are the saddest eyes, possessed by the most tragic character in the novel.