An old black man sitting on a couple of grocery crates playing the string guitar during the day.
Someone who is visible is more than fifteen years old.
An old man must be more than fifteen years old.
Old black man is someone more than fifteen years old.
"An old man" indicates someone who is more than fifteen years old. His actions of sitting and playing are described, and it's during the day so it is not dark out, so he must be visible.