I've been treating my canker sore with salt, alum and orajel. it has turned black. is that normal? why would that happen?

Healing canker sore: Good day; Some OTC canker sore medications use a mild cautery or caustics to deaden nerve endings and lessen symptoms. After a few days the dead tissue should flake off. Keep the area vaselined until this happens.

Canker Sores: are the most common oral lesions. Individual ulcers typically last 1-2 weeks. The condition is idiopathic (cause is not known) recurrent aphthous ulcers is usually self-limiting. Try to keep adequate oral hygiene as possible. If concerned see your dentist.

Tissue damage: Canker sores self-heal in 10 days to 2 weeks. Your well meaning but over-zealous attempts at self-treatment did damage to your tissues and may leave a scar. Please see your Dentist for care rather than a DIY approach.

DETAILS!!!!: Do you have brown or black skin? The darker your skin the more likely you will notice dark pigmentation after an injury or irritation or inflammation. This has a potential to slowly resolve with time.