Hello, Welcome to healthcare magic. Black patch on the lip could be due to a condition called as a fixed drug eruption. Did you take any medications when you were sick? Fixed drug eruption is an adverse reaction to a medication and rarely to food colorants and preservatives. It will recur every time you take the same medication. It is difficult to get rid of the coIour completely. It will slowly fade away with time. You could apply a moisturiser or lip balm to prevent chapping. Breathing through the mouth generally does not cause such a patch. The other possibility is a condition called as lichen planus. It is an allergy which responds to topical steroid formulation like mometasone. Hope this helps you. Take care.