let's not forget about the "blues"
Interesting point. Plenty of blues songs imply depression or insanity, without addressing them directly. And the older ones come from a world where the whole concept of "mental illness" was wrapped into other cultural points of reference.
Early this morning
When you knocked upon my door
Early this morning, oooo
When you knocked upon my door
And I said hello Satan
I believe it's time to go
Me and the Devil
Was walkin' side by side
Me and the Devil, woooo
Was walking side by side
And I'm going to beat my woman
'Til I get satisfied
She said you don't see why
That she would dog me 'round
(Spoken) Now baby you know you ain't doin' me right don'tcha
She say you don't see why, whoooo
That she would dog me 'round
It must-a be that old evil spirit
So deep down in the ground
You may bury my body
Down by the highway side
(Spoken) Baby, I don't care where you bury my body when I'm dead and gone
You may bury my body, woooo
Down by the highway side
So my old evil spirit
Can get a Greyhound bus and ride
Mental illness? Righteous anger at having been done wrong by a woman? Literal spiritual possession? Mental distress (accompanied by physical violence) is a trope in a lot of early blues songs, but the episodes are usually framed in a way that puts interpersonal relationships at the center of the narrative. I don't know of any that really give consideration to the underlying mental condition of the narrator, outside of how he is responding to a particular event. It's always "I've got the blues because ________", which in itself is a little window into what it was like to have clinical depression before the condition was understood.