Thanks for using Health Care magic. Read your query. Some medical professional do use silver nitrate to be applied for canker sore. It chemically cauterizes the nerve endings in the area and usually result is a potentially longer lasting ulcer but it doesn t hurt during the healing period.The swelling and the stain will take a around a week to resolve and get the normal tissue . Do salt water gargling and if painful ,you can apply dentogel /mucopain ointment to relieve symptoms. If the swelling and blackness is still persisting after a week or so ,please report back to your doctor. Hope this was helpful. Thanks and regards.