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.
Life, Liberty and the pursuit of Happiness is a well-known phrase in the United States Declaration of Independence. The phrase gives three examples of the unalienable rights which the Declaration says have been given to all human beings by their Creator, and which governments are created to protect.