Most of the blacks who arrived in Atlantic Canada were not slaves. They were former slaves from the southern colonies (in what is now the USA) who had taken refuge with the British forces during the American Revolution. About 3,000-3,500 such people were taken to Nova Scotia either during, or directly after, the war.

Pretty much exactly the same thing happened after the War of 1812, when circa 2,000 black refugees were settled in Nova Scotia and 400 in New Brunswick.

The British naval officers took the promises they had made to protect these fleeing slaves (who they had encouraged to flee to British protection) seriously. They resisted considerable pressure to transport them to freedom in British North America, rather than return them to their former owners.

Other possible sources of black settlement in Maritime Canada: there were a small number of slaves in these colonies until slavery became effectively illegal there in about 1800. There may also have been some escaped slaves who made there way there via the Underground Railway. There may also have been some small immigration from the West Indies, particularly after 1833, when slavery became abolished throughout the British Empire.

The largest immigration of blacks to the Maritimes, however, was the settlement of refugees after the American Revolution and the War of 1812.

Sources: http://en.wikipedia.org/wiki/Black_Refugees_(War_of_1812)

Http://www.novascotia.ca/Sam/virtual/africanns/