Blue is vast and pure, deep and clouded. The blue belt holds a dual meaning, this duality is intentional.
First the blue belt represents our physical abilities. Symbolic of the sky, the Blue belt serves as a reminder of the vastness for which we reach. As we grow we reach towards the heavens feeling they are within our grasp. In our self-absorbed state we forget that we see but a small portion of its vastness. We tend to ignore this vastness and focus only on what we see and know as our skills advance. This leads to a state of over confidence. Instead of mastery you have only taken a sip of what is you yet have to learn! Over-confidence leads us to loose our humility, our humbleness, the Blue belt should remind us of our insignificance and of how much more we need to grow.
Second the blue belt represents our potential for mental depth and maturity. Here often the Sea is used symbolically for its depths and currents are difficult to see and harder to control. Only through determination can we advance here for introspection is a difficult task. As our physical strength grows through training, our mental depth and maturity should develop, strengthening our mind and our person as a whole. The lessons of the blue belt are that the strains of physical training are temporary, but mastery of self can last a lifetime.