Blackish gumline (supporting the tooth).

That would definitely be unsettling to notice, especially when it has appeared during chemotherapy. Oral pigmentation can occur with some chemotherapy medicines, although it seems to be an uncommon side effect and the exact cause can depend on the drug being used.

Since your oncologist has already seen it, it may be worth having a dentist look at the gumline as well, particularly if the colour changes further, becomes sore, starts bleeding, or develops into a patch or ulcer. Keeping the mouth clean but being gentle with the gums is also important during treatment. If you need to find a local dental service around the Sutherland Shire, the website for Shire Doctors and Dentists may be useful for checking the available dental services. I’d also mention any new mouth changes at each oncology appointment so the treatment team can keep an eye on them.