In Rom 4:6-8, David is used as an example of a man who had a virbant relationship with God.  The man who is blessed is one who walks by faith.  Thy know their faith in the Lord is the foundation for righteousness and not works.  You are blessed for two reasons.  First, your transgressions have been forgiven.  Secondly, you sins have been forgiven.  What a wonderful truth.  You are blessed because your sins will never be held against you.  You have forgiveness, peace with God and never have your sin being brought back to your attention.  If you feel guilty because of sin and you have repented then you know it is the devil who is trying to condemn you.  God takes our sin and tosses them as far as the east is from the west.  Walk in freedom not condemnation.