Well, the notion I had that it represented a subservient way of addressing someone IS what threw me - I’m white and they are black, and it seemed like a ‘throwback’ and made me uncomfortable.
But as time went on, I realized that however it may have begun, it is now a simple form of respect, and largely for age - as I said, I encounter it most as a way of addressing females, but especially older ones; and they use it without regard to color. It seems to be a form of respect when the relationship is too fond/familiar for formality, but one still wants to show respect.
I come from a Southern family - Virginia, with extensions in South Carolina; but I’ve spent my whole life in the DC area. The closest other form to this that I’ve encountered is when children are taught to refer to someone very close as Aunt or Uncle, when no such blood relationship exists but the person is older and respect for age is required from the kiddos.
The "AUNT"/"UNCLE" thing is still used, for adult family friends in South Africa, amongst the Boers. I don't know if it is commonly used by blacks there.
My progeny called GODparents "Aunt/s and Uncle/s", though there was no blood relationship.
Now, if you want to go back a couple of centuries, Southern whites called some black slaves Aunt or Uncle...a la UNCLE REMUS, who was based on a REAL person and his stories.