After washing my pan with dishwashing liquid, I was wiping it dry with a kitchen tissue and noticed black stains on it. So I washed the pan again with dishwashing liquid and wiped it but the black stains would still show up on the tissue. Also, in the areas of the pan (the exterior) which were only wet and had no dishwashing liquid used on it when wiping it also turned the tissue black.
The pan:
The kitchen tissue: