The color blue is used often in Angels and Shadows to convey strong spirituality. Often in popular culture, when we think of blue, it is used to express depression or loneliness. I have personally always thought of blue to be a calming, spiritual, and arousing color. It sparks feeling of peace and wholeness. Slatkin's trail as he is following the hunters is a blue trail. That is the color of Galan's soul. Galan isn't like other hunters. He was raised mostly by his Astrid mother, a woman of strong ties to the spirit world. Despite the cruel harshness of his father's temper, he retains her teachings at the risk of losing his own life to the other hunters.
In the book other characters have blue soul colors. Slatkin's is a dark blue. Most Orostiro have blue souls. It is the color used in representation of Saigolai. Like water, and a clear sky, it represents life. It is also the color of Celeste and Alexandria's homeland. Blue and White are used together in the kingdom of Trealon. It is a warm land, thriving at a sea port. The blue is used to represent the nature of that land; a comforting and prosperous land of peace.