What is the stall angle in an inverted flight configuration? How does it relate to the stall angle in normal flight?
As visible in the image below, in upright flight the lower pressure is on the extrados of the wing. In inverted flight the lower pressure is on the intrados.
The air stream separation at the stall will occur on a side with different characteristics. One could expect the stall will happen differently.
Equivalent representation, where the gravity is inverted and the wing orientation remains in the same direction. Flying inverted implies flying at a negative angle of attack.
The values taken into account are the airflow direction and the chord line, reflected in the angle of attack value.
As commented, while the two previous images suggest a wing in horizontal flight, the situation can be extrapolated to any stable linear trajectory with a negative AoA.
The horizontal plan or the pitch angle are not necessary to determine the stall angle (though they influence the stall speed).