Someone who hurt you w/o a care on how you felt. Left you alone. Left you at your lowest. Basically someone/something or some situation that hurt you so bad that changed you forever.
Now you are numb. Now have trust issues/commitment issues/push people away and etc.
And think you are alone no matter if others tell you otherwise.
Your blue is someone/something that changed or hurt you forever...
someone who hurt you; someone who broke you. yet you can't seem to get over them/let them go. because even though they hurt us the most, they are simply unforgettable.
"Hey, you knowAl?"
"Yeah, I know him. He's my blue."