@slim mentioned in this answer that:
In Blues music for example, some notes are deliberately flattened -- but others are not, and a listener with experience of the Blues would still spot badly pitched notes.
While I have long been aware of flattening and partial bends in blues, I had not realised this was a standard part of blues and can't seem to find any definitive description or instruction.
So my question is:
When will notes be flattened, and by how much, in the blues tradition?