He could gain the ability to sense and manipulate the fear of others. Since Tartarus is a place full of horror, the blessing might give Percy an edge in battle by being able to use his enemies' fear against them. For example, he could create illusions based on their worst nightmares.
His water - based powers might become corrupted. Instead of just controlling clean, life - giving water, he could now also manipulate dark, poisonous waters from the depths of Tartarus. This would make his powers more dangerous but also more unpredictable.
His water - based powers might intensify as he is in a place filled with so much negative energy. He could learn to control the strange, dark waters in Tartarus, which are probably more powerful and unpredictable than normal water. Maybe he can use them as a shield or a weapon in ways he never thought of before.
His powers would likely be much darker. He might have the ability to manipulate shadows like Nyx, using them to hide or even attack. And from Tartarus, he could have the power to cause earthquakes or open up chasms in the earth. He could also have a natural resistance to the harmful effects of the Underworld, such as the poisonous gases and the draining of energy that normal demigods experience.
His powers might become more related to the night. He could gain the ability to see clearly in the dark, like having night vision. Maybe he could also manipulate shadows, using them as a form of defense or offense. For example, he could create shadowy constructs to fight his enemies.
His powers could also be more regulated. The Roman military has a strict code of conduct, so Percy may not be able to use his powers as freely as he did as a Greek demigod. He might have to get permission from his superiors before using his powers in combat or for other tasks. This could lead to some interesting situations where he has to hold back or find alternative ways to solve problems.
His powers would likely become more god - like. He might gain the ability to control lightning, like Zeus. Also, his strength and speed could be enhanced to a much greater degree. His control over water, which was his main power before, could also be amplified and perhaps combined with his new lightning powers in some unique ways.
If Percy is a descendant of Kaguya, his powers might change in a way that combines his demigod abilities with some unique Japanese - inspired powers. For example, he could gain the power of illusions, like Kaguya was said to have in some stories. This would be a new and unexpected addition to his combat and survival skills. He could use these illusions to deceive enemies or find hidden paths. Also, his speed and agility might increase, as Kaguya was often depicted as a very graceful and swift figure.
His powers might become more related to Hera's domain. For example, he could gain some control over marriage and family bonds. He might be able to sense when relationships are in trouble or strengthen the loyalty between people. Also, his existing powers could be enhanced in a way that reflects Hera's influence. Maybe his water - based powers could be more refined and have a new aspect to them, like being able to purify water to heal emotional wounds as well as physical ones, since Hera is associated with purity in some ways.
With his friends at Camp Half - Blood, Percy might become a bit of an outlier. His connection to Zeus through the blessing could make some of his friends who are more closely tied to Poseidon or other gods feel a bit uneasy. However, it could also earn him respect from those who admire Zeus' power. Percy would have to work hard to maintain his friendships while dealing with this new dynamic.
His powers could become more ice - related. Since Khione is the goddess of snow, Percy might gain the ability to freeze water instantly. For instance, instead of just controlling water, he could turn it into ice sculptures or use ice as a weapon, like creating ice shards to throw at enemies.
His physical strength might increase. Thor is known for his great strength, and as his adopted son, Percy could inherit some of that. He might be able to lift heavier objects or fight more powerfully in close - combat situations. Maybe he could even hold Mjolnir for a short period of time, which would be a huge boost to his powers and status in both the Greek and Asgardian worlds.