Toothpaste is never a great idea for skin since it can cause burns. However, it sounds like the darkening is just a sort of scarring the healing process has left behind, rather than a scar due to burning. With time, it may fade away more. You could also try an anti-pigmentation or scar gels etc. A dermatologist may be the best to suggest something.
Aloe vera gel (pure) is great with reducing scars.
All the best!