Final answer:
A bruise appears black and blue due to broken blood vessels in the dermis that leak blood, with the color being the result of breakdown pigments like biliverdin from the destruction of hemoglobin.
Step-by-step explanation:
A bruise turns "black and blue" due to broken blood vessels in the dermis. When these vessels are damaged, blood leaks out into the surrounding tissue. The color change associated with bruises is from the breakdown pigments formed from the destruction of hemoglobin. At the site of an injury, biliverdin from damaged red blood cells (RBCs) produces the greenish hue, while hemoglobin in the pooled blood can be responsible for the blue-black color. Over time, these pigments break down further, and a bruise can change in color from blue to greenish-yellow and finally to a yellow-brown before healing completely.