I always thought she was blonde natuarlly but I saw some really old clips on YouTube of her from the 1960s and she appears to have dark hair. Bad clips or good dye job. I imagine now that she’s old she dyes her hair but did she in the hey day of the 70s?
She’s natural. Trust me 
Joe
Cite? 
I don’t have a cite, but I do have a story. Back when ONJ was about 14 or 15, I knew a girl going to the same high school as her. She told me about ONJ singing in some coffee shop in Melbourne, and we talked about going to hear her, but we never did. So, I never have met ONJ, even though I had heard of her long before she was famous, and I don’t know her natural hair colour.
(ETA: But I did know her father very well, and he was not blond.)
How about their (singing) postman?
She undoubtedly was a natural blonde as a kid. I was a very fair blonde but it darkened by my teenage years and now is actually almost black. Well, and grey.
I can’t find any childhood photos, but at the age of 18, her age in the first photo below, she was a light brown. She’s fooled you all these years thanks to talented hairdressers.
http://www.onlyolivia.com/onj.html