The high priest tore his clothes after hearing the words of Jesus. This was a legal act that signaled a guilty verdict and showed that no more witnesses were needed. The rest of the Sanhedrin agreed that Jesus was guilty and deserved death.
It is not completely clear what the leaders considered to be blasphemy (showing disrespect toward God) in what Jesus said. It might have been his statement "I am," which is how God referred to himself in Exodus 3:14 (see John 8:58). However, Mark may not be repeating the exact words Jesus spoke at the trial (compare Matthew 26:64; Luke 22:70). The phrase could simply mean that Jesus was answering “yes” to the high priest’s question. He was the Messiah, the Son of God.
During the second Jewish revolt against Rome, Bar Kokhba (AD 132–135) claimed to be the Messiah, and this was not treated as blasphemy. Jesus calling himself the Son of Man cannot have been considered blasphemous by itself. Before his trial, Jesus had used this title many times, and no charge of blasphemy had been raised.
However, the high priest and the Sanhedrin reacted strongly when Jesus identified himself as the Son of Man from Daniel 7:13. This indicated he was coming in God's name to judge the world.
This declaration came after:
Jesus had already forgiven sins (Mark 2:5–7; Luke 7:48–50),
claimed to be the Son of God (Mark 12:6),
pronounced judgment on the temple (14:58), and
claimed to be Lord of the Sabbath (2:28).
The religious leaders had already decided that Jesus should die. They now announced the guilty verdict they had already planned (14:55).