A Pirate Flag of the "golden age of piracy" would typically have been made from silk. Silk flies well and would have been plundered from the Spaniards.
The Jolle Rouge, later to be known as the Jolly Roger, was dyed with blood.
The silk would have been light colored, and after the blood has dried and aged, it appears black.
It is possible to draw your design on the silk using wax, then when it is dyed in blood, the design remains light colored.
Obviously, in today's world animal blood would be used, contact your local butcher, he should be able to supply what you need here.
Following the dying processing , allow it to dry thoroughly in the sun. light rinsing to get out the excess and then washing in hot water to soften and remove the wax.
I'm sure there are other methods, but you asked for authentic.
Pirate