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.
Our safety, our liberty, depends upon preserving the Constitution of the United States as our fathers made it inviolate. The people of the United States are the rightful masters of both Congress and the courts, not to overthrow the Constitution, but to overthrow the men who pervert the Constitution.