The US was not one of the initial signatories of the 1856 Declaration of Paris which outlawed privateering, and the Confederate Constitution authorized use of privateers. 

During the Civil War Confederate President Jefferson Davis issued Letters of Marque to anyone who would employ their ship to either 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 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.