Blue stain is a type of discoloration that occurs in wood, primarily caused by certain fungi. These fungi thrive in damp conditions and can penetrate the wood, leading to a blue or grayish hue. While blue stain does not typically affect the structural integrity of the wood, it can impact its aesthetic appeal. Commonly found in species like pine and spruce, blue stain is often seen in lumber that has been improperly stored or is exposed to moisture. It is important for woodworkers and builders to recognize blue stain, as it may influence the choice of materials for specific projects.