In Aussie slang to call someone "Blue" or "Bluey" means they've got red hair.
In the early European migration days (convict days) a lot of those people
were of Celtic origin with a significant proportiion of redheads.
So to call someone "True Blue" means that they could trace their origins
back to a convict arrival. It's close to the best compliment that you
can give to an Aussie.
Thanks mate, I really appreciate your help. You're true blue.