Beyonders: A World Without Heroes! by Brandon Mull.
Jason leads a relatively normal life until one day at the zoo, when he notices strange music coming from the mouth of a hippo. While leaning over the hippo tank's guard rail, he falls in. The hippo swallows him, but instead of being digested, he's transported to a different world called Lyrian.
After wandering a short way, he learns that the group of musicians playing the music that he heard are called the Giddy Nine and that they plan to ride a raft over a nearby waterfall. In an attempt to save them, he talks to the rescue squad present and asks them why they are not rescuing the band. The archer states that they are just there so that if someone decides to turn back they can help. Before the raft reaches the end of the waterfall, Jason pushes the little man with one arm into the river and grabs the archer's bow with an arrow attached to a rescue line. He aims to shoot the line onto the boat in order to give the band something to grab onto, but instead, his shot goes awry and the arrow pierces the shoulder of one of the musicians. One musician is able to jump to shore before the line is cut by the archer but the rest plummet off the edge of the waterfall. The archer had helped the little man out the river so the archer and the little man who was part of the rescue squad pursue Jason for interfering, so Jason runs off into the woods.
The loremaster then informs Jason that he's sealed his own fate, and must leave immediately in search of someone called the blind king. After being given directions to the blind king as well as some food, Jason leaves the repository of learning.
Found with a search for fantasy novel hippopotamus "blind King"