Perhaps an old member of a forgotten clan who was exiled because of his dark blood. He returns with a vendetta against the village that shunned him. He sees Naruto as a symbol of everything he hates and tries to destroy him. The main antagonist could also manipulate other ninja to turn against Naruto by spreading fear about his dark blood.
The main antagonist in a 'dark blood Naruto fanfic' might be a mysterious figure from a different land who covets the power of the dark blood. He sends his minions to Konoha to steal the secrets of the dark blood. When Naruto gets in his way, he becomes the prime target. This antagonist could have his own dark and complex backstory that led him to seek the dark blood power.
Might be one of the powerful guards who has a grudge against Naruto for some reason. Maybe he believes Naruto is a threat to the order in the prison and tries to get rid of him secretly.
The main antagonist could be the Hidden Leaf Village itself. Since Naruto was abandoned by Kushina, the village that failed to take proper care of him could be seen as the enemy. The village elders or those in power who turned a blind eye to his suffering could be the ones he has to oppose.
It could be Orochimaru from the Naruto universe. He might have found a way to cross over to the Teen Titans' world and has a new plan to use Naruto's dark side for his own evil purposes.
It could be an unknown ninja who has been secretly studying dark energon for years. He wants to use it to take over the ninja world, believing that the power of dark energon will make him invincible.
It could be a rogue Dark Jedi who has escaped from his own Order and sees the Naruto world as a new place to spread chaos. He might be seeking powerful chakra sources to fuel his Dark Side powers.
It could be Sasuke. If Sasuke continues to pursue his own vendetta in a more extreme way and Naruto's attempts to bring him back fail, Sasuke could become the main antagonist. Their rivalry could turn into a full - blown battle for power and ideology, with Naruto being pushed to his limits and potentially turning dark in the process of trying to stop Sasuke.
It could be the warden of the Blood Prison. He might be a corrupt figure who is using the prison for his own gain. He sees Naruto as a threat to his operations and tries to keep him locked up at all costs.
It could be one of the powerful and corrupt prison guards. They might have their own hidden agendas and see Naruto as a threat to their power or secrets within the Blood Prison. So, they start a scheme to make it seem like Naruto has betrayed everyone.
It could be a rogue ninja who has mastered the dark side of Blood Blockade Battlefront techniques. He might be seeking power to take over the ninja world and uses his knowledge of this unique technology to create chaos.
The warden of the Blood Prison could be the main antagonist. He might be a power - hungry and corrupt individual who sees Naruto as a threat to his control over the prison. So, he devises plans to get rid of Naruto, which ultimately lead to Naruto's death.