The U.S. was not one of the initial signatories of the 1856 Declaration of Paris which outlawed privateering. This allowed the Confederate Constitution  to authorize use of privateers. 

During the Civil War Confederate President Jefferson Davis issued Letters of Marque to anyone who would employ their ship to attack Union shipping or bring badly needed supplies through the Union blockade into southern ports. Many of the supplies brought into the Confederacy were carried aboard privately owned vessels. Blockade runners were specifically designed and built to outrun Union ships on blockade patrol. Ships of this size and configuration were the workhorses of the merchant trade during the mid to latter 1800's. Today we "keep ta the law" and our modern Letters of Marque are the Captain's USCG papers, insurance proof and the like. "Just so's ye know"......