As I understand it, once the British outlawed the Slave Trade they used the British Navy (at a bit of a loose end after the Napoleonic Wars were done)to put a blockade on slavers, especially on the West Coast of Africa, who were still trying to take slaves to Brazil and the United Staes (this was long before the Confederacy of course).

American ships trying to run the blckade were treated the same as any other ships, and seen essentially as pirates. Fair enough, they were half the time, and in any case they were in an even dirtier trade.