I'm going to guess from this question that it was posed by your teacher and you are to write about something personal.  This means that any Enotes answers could best be used as inspiration or to help you get your ideas flowing.  In the end, you should probably write about your own experience.

That said, I encourage you to think about a time when you felt particularly honest, passive, peaceful, or sentimental.  For me, this combination of feelings often comes after something very stressful has ended.  For example, after running my first marathon, I was physically exhausted in a way I had never experienced before, but I was also emotionally overwhelmed as this was one of the biggest accomplishments (physically) of my life.  I cried when I crossed the finish line (as many do).  In the hours that followed, a strange sense of malaise came over my body.  I was not tired in a way that made me want to sleep, but instead I just had this pervasive sense of contentment.

After training for 4 months to do something no one expected me to be able to complete, and then completing it, I was just filled with happiness and pride.  I remember for the next few weeks just being in a place of confidence in myself.  Of course I slept really well at night, but not simply out of physical exhaustion.  It was an opportunity to be introspective and somehow I had this feeling that I could let go of unrealistic expectations that others put on me.  It is weird to admit that this physical challenge had such a profound spiritual effect on me and allowed me to let go of stress in many areas of my life.

I do not, however, believe that my experience is unique.  I think in many ways our bodies sychronize physically, emotionally, and mentally more often than we realize.  Perhaps for you there is a time when you underwent an experience that was so stressful you wondered how you would get through it.  How did you feel the moment you realized it was over?  To me, this is the moment that the "color blue" feelings are most prevalent.