Blue is not technically fallen, but got his "soul" sacrificed in the place of a higher-ranking angel and thus behaves like one. He knew the one who ruined him, and now lacks the energy even to abhor him.
The angel who offered Blue's soul as an alternative had a grudge against him, held for aeons and festering until it ultimately culminated in the erasure of what Blue once was. He had invited another's devotion, and apparently unduly so; it may have beenn act of malevolence, in the end, but there is no judge to declare such things, for the great haven cares not what falls and the victim of this abhorrent act has himself fallen into such apathy that he is ignorant.
He doesn't remember his own name, and it has been erased from the minds of those who once knew him, replaced only by a void of uncertainty and anger. An angel's name has power; it's who they are; it is one of the highest truths about themselves, and so forging a new name and leaving behind the forgotten is not enough. There is nothing that can expedite his convalescence—it's uncertain, indeed, that it will ever approach. What is encroaching, however, is the erosion of the muted emotions he was permitted to feel in the first place; he can no longer feel joy, pain, fear...he stands unfazed by all, that stolid and uninterested sentry, waiting.
While he's currently unable to express or really feel emotion except in the most compelling of circumstances, he is still giving, and most of his acts are either caring or an approximate. He is, however, too lethargic and indolent for this to be particularly evident to most of his interlocutors.
It's possible to get him mad, but since he emulates a very particular kind of fallen he can't really hurt you. There is, however, someone who can...
— The black on his wings is a sign of his soulless status, and carries with it the stigma of a crime that was committed by another. Usually he is shunned by angels for this, and because he cannot fight back he is a popular target for violence when his reputation doesn't precede him—yet, paradoxically, should ever the ostracization carry over into conflict, usually it is the other that does not escape unscathed.