Eh..?That is true.Soul gem itself is the soul of a magical girl.That is correct.Madoka.If the soul gem turns black, the heart also turns black, and the fate that lies at the end of it is... remains the same.The "anti-witching" ability expressed in Sayaka is never "not becoming a witch".Still, as long as certain conditions are met.It wouldn't have led to witchcraft, but...Perhaps it was only an unfortunate fate for Sayaka that you showed up here.