The Black Bird, The Dark Slope Lyrics
I think it's pretty evident the blackbird is a metaphor for depression, sitting inside his chest and causing great pain. He's trying to bear it, but eventually it starts to show through (Its wingtips push across my lungs and fill them full of feathers/Now they poke between my teeth and that's why I thirst).
Finally, when he's just given up, he asks that at least the bird be killed or driven out, regardless of how it may harm him, as the bird has just been destroying him and his life for so long at this point that he doesn't care anymore. But then he's also acknowledging that he has said some things during this depression, or maybe will later, or possibly is referring to something he wrote (a suicide note?) when talking about the bird writing. He's saying that it wasn't really him saying any of those things, it was all the bird, the depression.
I think anyone who has ever struggled with depression can related very well to this song. For starters "The black bird sits atop my guts and spreads its wings for flight" is most likely refering to the stomach aches and loss of appetite common in depression. As the depression begins to get worse it leads the author to do and say things they would normally not do leaving them empty and alone "when he flies me to the top there's nothing but the fog." The heart of stone is the cold hopelessness and eggshell bones are the instability that comes with being depressed leading you to become lost from a sense of self and solid ground. It is in the end a general slope of mental decline. "The black bird dips its beak in blood and writes its thoughts in cursive 'cross The bones that are its jailer and my ribcage And when you turn me inside out, believe in me without a doubt The words were all of his and none of mine" I think this could be referring to either a suicide note that was written purely out of crippling depression, or possibly words of spite the author may have said to someone that they now regret saying and maintain these words as their biggest regret but beg either way that the reader hold true this is not the actual opinion/ mindset of the author but the depression driving them mad.