To be blackened means to become darkened or charred, often as a result of burning. It can also describe the appearance of blood gathering and spreading beneath the skin in a way that makes it appear darker.
Charred vegetables on a grill can be described as blackened from the high heat. Similarly, a piece of toast that is forgotten in the toaster until it's nearly burnt is also blackened. This word can also describe surfaces that have been darkened by smoke or soot, such as the walls of a fireplace. Things can become blackened as a result of cooking, fire, or even age, giving objects a dark, charred appearance.