There are several processes that occur in alchemy, and this image refers to "the blackening"...  The blackening is a necessary part of the process.  We start out innocent, good natured, trusting... and somewhere on our path we are led into somewhere dark, where we are then wounded, and we suffer an immense transformation, that is a death of the person we used to be.  A death of our naeviete.  When this happens, we feel as though we are too wounded to keep going.  The blackening takes our innocence, and transforms us into a more powerful being.  A being who is capable of defending against future attacks, a being able to see danger before it has arrived on her doorstep.  In order to move through a world full or darkness and light, we must be initiated into the darkness once, so that we will transmute our innocence into fierceness, so that the darkness cannot enter us ever again.  We chemicalize the darkness out of us.  It is a purification, but first we have to go through the darkness stage.