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.
Secure is used to mean make safe, as in keep from being blown loose and being lost, as in 'secure the hatch!. Blessings mean the truly good aspects, the reasons to have [and fight for] liberty.