The story
From black + Dharug dyin (“woman”) (David Nash, "Placenames evidence for NSW pidgin" in Felicity Meakins, Carmel O'Shannessy (eds) Loss and Renewal: Australian Languages Since Colonization, Walter de Gruyter, 2016, p.133.) https://books.google.ca/books?id=spjUCwAAQBAJ&pg=PA133&lpg=PA133&dq=%22blackgin%22&source=bl&ots=PAH4wZMnI1&sig=6pQWiSTFpQV6FcU9ZFhRzaTkbbQ&hl=en&sa=X&ved=0ahUKEwim-YfEg9vMAhUK-GMKHccqA_0Q6AEI5AEwLw#v=onepage&q=%22blackgin%22&f=false
Etymology from Wiktionary, CC BY-SA 4.0.