We put "sad" first because, as we discuss in "What's Up with the Title?" a "blue period" is to be taken seriously. But, if taken too seriously "blue" can become a permanent condition. One of this novel's main points is that it's OK to laugh and cry at the same time. Laughter can be a powerful tool in dealing with grief and tragedy.
Much of our laughter is at the expense of young Jean who bumbles confusedly and naively through his mock adventure. Like many people in the middle of a tragic time, what's going on in his personal life overshadows the news of World War II. (This may be why we don't hear anything of it.) Jean's naiveté is also endearing. We can't help but admire his willingness to plunge into new situations and to reach beyond his experience. Jean doesn't let anything stand in his way when he wants something. This isn't always good for him. His pushiness with Sister Irma (and suggestion that she devote herself to art) might have caused real problems for her at the convent. Nuns are supposed to devote themselves to God, not art. You can argue that to young Jean, there is little difference between religion and art.
This is where the confessional and ironic aspects of the story comes into the picture. "Blue Period" uses dramatic irony – the older narrator and the reader pass judgment on the young Jean, who confesses to the "crimes" of his youth. He hints that he regrets some of his treatment of Bobby, as well as his cruelty towards some of his students. He seems to be confessing his obsession with Sister Irma, and even his obsession with his mother. We think he's being a little too hard on his young self, but realize that this is necessary to produce both the tragic and the comic effects of the story. The fact that Jean regrets his actions adds another layer of tragedy to the story. Plus, the regretted actions are so extreme that we can't help laughing at them as we feel the sadness.