Is it safe to consume the mushroom when the inner part of the mushroom is dark?
Yes. Different from preserved mushrooms, that is always submitted industrial processes including preserving chemicals, those are generally harvested young and do not have this dark inner part. The fresh mushroom (in natura) may be harvested in many different stages of growth and when becoming “mature” may present the development of the gills, the dark part. This can be perfectly consumed, and, in some countries, they are even preferred as it brings a more intense flavor.