> In 499 BC, he shaved the head of his most trusted slave, tattooed a message on his head, and then waited for his hair to grow back. The slave was then sent to Aristagoras, who was instructed to shave the slave's head again and read the message, which told him to revolt against the Persians.

Since this was a trusted slave, the tattoo seems unnecessary. The slave could just tell Aristagoras "Histiaeus says to revolt against the Persians".

The fact that the slave had been signed confirmed the authenticity to the recipient.

There is trust and there is trust.

There was enough trust that the slave did not desert and that they were taking the message to the destination.

There was not enough trust that the slave would actually know the CONTENT of the message. As such the slave wouldn't even know to where to desert to :)