This is a Latin term and was used as a hymn title which literally means "Blessed Sin." The idea is that without sin and evil we would never experience some of the greatest attributes of God. Having fallen to sin and its power Adam brought a blight and a curse to man. God shows man mercy, compassion, forgiveness, patience, grace, longsuffering, etc. God not only displays these attributes, but displays them to a degree that is overwhelming and in a way that is pragmatic and personal. We learn his person in his comfort.
Assignment: Read the two Petrine epistles and make a list of the attributes that God has that can be only displayed to a sinner or one being hurt by evil.