Hello,
Normally, we can injure our gums just like any other part of your body. Example are Falling on your face, eating something with sharp edges, and even brushing or flossing your teeth too hard can bruise your gums. Bruises on the gums are usually dark red or purple, but they can also be dark brown or black. You may also have some minor bleeding and pain in addition to the bruise. Bruises usually heal on their own without medical treatment.