US Secret Service - the United States intelligence agency that protects current and former presidents and vice presidents and their immediate families and protects distinguished foreign visitors; detects and apprehends counterfeiters; suppresses forgery of government securities and documents.
It's  blessings not blessing.. The entire clause is ...to secure the  blessings of liberty to ourselves and our posterity....    The blessings of liberty refers to all of the benefits that are  afforded by having liberty.
And the peace of God, which passeth all understanding, will keep your hearts and minds through Christ Jesus. Weymouth New Testament. And then the peace of God, which transcends all our powers of thought, will be a garrison to guard your hearts and minds in union with Christ Jesus.