Pitch is Jack's enemy, or at least he becomes it during the movie. It seems mostly that Pitch just ignored Jack, because he wasn't important in his schemes, aside he seemed to see him as a neutral party and someone who isn't really a threat to his plans.
Then after Frost shows that he actually is someone who could defeat him, after he takes down Sandman, Pitch's first idea is to try and get Jack on his side, playing the lonely card and the fact that Pitch knows how it feels not to be believed in and invisible to everyone. Even if for a moment it seems that Jack will take the bite, because in a certain sense he understands Pitch's want for being seen, he doesn't share the idea of how, Jack doesn't want to be feared, he wants to be loved and have fun with his believers, he doesn't want them to cry and hide when they see him; so he refuses, and here he does the last step and becomes the enemy of Pitch Black, and the Boogeyman acts according to his ways - he uses someone Jack loves to push him in to the corner.
So in a certain sense their relationship is quite complicated, because Pitch
sees Jack's potential and would love to have him at his side, and also as the
master of the fear, he knows also Jack's fear of being left alone and being
invisible, but he doesn't know, because of what he is, that Jack has a big heart
and he is full of love, aside to be loyal. He does see a glimpse of it, when
Frost to save Baby Tooth is prepared to give up his most prized possession: