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.
Quotations. I know not what course others may take; but as for me, give me liberty, or give me death! [Patrick Henry]. Liberty is liberty, not equality or fairness or justice or human happiness or a quiet conscience [Isaiah Berlin Two Concepts of Liberty ]. The tree of liberty must be refreshed from time to time with the blood of patriots and tyrants. It is its natural manure [Thomas Jefferson]. Liberty is precious-so precious that it must be rationed [Lenin]. Liberty means responsibility. That is why most men dread it [George Bernard Shaw Man and Superman ].