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Was the Son of David expected to bring mercy?

Matthew, Mark, and Luke record blind men calling out to Jesus. For example (NET):

27 As Jesus went on from there, two blind men followed him, shouting, “Have mercy on us, Son of David!” (Matthew 9:27 similar 20:29-31 Mark 10:46-48, Luke 18:35-39)

In each case the blind see and call upon Jesus as the Son of David. While they eventually ask for and receive their sight, their immediate request is for mercy.

What is the reason the blind are seeking mercy specifically from the Son of David? Is there a Scriptural basis for looking to the Son of David as bringing mercy?