There are two aspects to Blue Like Jazz that make it tricky fare for a movie. The first is its focus on faith. Generally, movies approach this subject in one of two ways: either to dismiss or caricature its adherents, or to make them unrealistic heroes in something much closer to a Hallmark special. And right there, I just dismissed and caricatured a genre of movies (mea culpa). But that's the challenge with film, is that we have so few examples of people genuinely wrestling with the subject of faith and belief in a realistic setting. Blue Like Jazz at least strives to find that balance, where realism and earnestness intersect. Don's journey is messy, and involves a lot of language and settings that, quite frankly, will not play well in the conservative Christian crowds of his backstory. And on the flipside, he genuinely wrestles with his faith, asking and fielding the tough questions about God and belief that most people struggle with at some point. His is no longer a cloistered belief system untouched by the realities of history and modern thought. It's more of a painful cut on an increasingly tender wound.
But the other aspect of Blue Like Jazz is that it's based on a book. And although normally that yields more than ample fodder for a film, here it seems to have yielded too little. The main issue is that the book isn't really a single narrative, or at least the kind that would normally service a script. It's more of a journal, some of which contains stories from a similar life and character, but just at often its populated with musings and reflections on the state of belief in America. And while it yields the potential for a good character, it is often under-realized in this particular film.
Allman's Don encounters and deals with a lot, but he often does it from a fairly passive position. The other characters around him are much more dynamic, often forcing his encounters at him. In fact, he is regrettably more milquetoast than hero, making it difficult to care too much about him as a character. Likewise, a bit too much is made of "look how crazy things are at Reed!" It's a schtick that drags on much too long during the course of the film. Not only that, but it actually cuts into the opportunity of making Don a more layered and interesting character, as opposed to relying on the contrast between his previous and current settings.