The dark influences have been a double edged sword. It is a sword that harry has walked the edge of and on more than one accasion he has overbalanced into the darkness. His dance on the razor's edge has made him strong. Strong enough to resist incredible temptations.
He has drifted into red light saber territory, as detailed in the original post and on other instances. He has also resisted temptation to grasp power more extensively and more consistently than most if not all mortals. I do not think the Harry Dresden of Storm Front could have resisted the calls of Laschiel in DB and certainly not
.
There is evidence Harry has been groomed as a weapon, and some of this grooming has made him darker and "stained". It has also forged him like a sword. A magical sword with a high ego, able to assert his own will and defy anyone who would subvert that will. Harry's story is the balancing act between the light and the dark. The fate of the world hangs in the balance.