Although Peter Blood is a man of integrity, he is wrongly accused, convicted of treason, and sold off into slavery in Barbados. Yet despite this horrible beginning, the theme of the book is that justice will come in the end to a man wrongly punished as long as he hangs on to his honor and integrity.
In part, this is because the novel wants to show that poetic justice is at work in the universe. Poetic justice is a term that means that justice will, one way or another, come to all people. It is premised on the idea that this is a just universe. So, for example, if a person murders someone, even if the law finds him innocent, he will nevertheless come to a bad end. In the novel, coincidences become the vehicle for poetic justice.
Blood, for example, is able to escape slavery and become a pirate. However, even though the English have wronged him, Blood maintains his honor and never attacks British ships. In the end, poetic justice is achieved when Blood is pardoned and wins the hand of Arabella.