At the start of the American Revolutionary War in April 1775, the population of the American colonies was approximately 1.5 million. Of these at least 300,000 were black slaves, mainly in the south. 
On November 14, 1775, John Murray, Earl of Dunmore and Royal Governor of the Colony of Virginia, issued a proclamation offering freedom to slaves who would leave their masters and join the British side. That proclamation declared, in part, “all indentured Servants, Negroes, or others, (appertaining to Rebels,) free that are able and willing to bear Arms, they joining His MAJESTY’S Troops as soon as may be.” 
By 1776 the Dunmore Proclamation became general British policy throughout the colonies, and in 1779 Sir Henry Clinton, who was a top British General, issued the Philipsburg Proclamation expanding Dunmore’s Proclamation to include any rebel slave who could escape, ready to serve for the British or not, anywhere in the colonies.
Although only an estimated 800 slaves immediately joined the British in Virginia as a result of the Dunmore Proclamation, eventually as many as 30,000 slaves throughout the colonies did so and worked as soldiers, laborers, pilots, cooks, and musicians for the British.
In the final battle of the War, the Battle of Yorktown, in October 1781, the British were defeated, and British General Cornwallis surrendered and thereby abandoned hundreds of black soldiers to the Americans for a return to slavery.
By the winter of the next year (1782), it had become clear that the British would soon have to evacuate the American colonies. At the time thousands of Loyalists were in the British-held strongholds of New York, Charleston, and Savannah. All Loyalists knew that staying in the new country invited retaliation against them by the victorious Americans, and as a result many left the colonies.
The Black Loyalists were at the even greater risk of being returned to slavery and subjected to cruel punishment for having escaped. Indeed, the terms of the Treaty of Paris ending the War required the British to return the former slaves to their owners.
When those treaty terms became widely known in the colonies, many white slave-owners and their agents from the southern states went to New York City to kidnap and seize their former slaves in anticipation of the signing of the treaty. In addition, the British abandoned some of the Black Loyalists to the Americans or sold them in the West Indies or traded them for White Loyalist prisoners.
When the War formally ended with the signing of the Treaty of Paris on September 3, 1783, the British were nearing the end of their seven-year occupation of New York City (then only the southern end of Manhattan). Thereafter, over the next three months, the British evacuated more than 29,000 military personnel, Loyalists and liberated slaves from the City although the Treaty of Paris required the British to return the slaves to their owners.
Among those evacuees were 3,000 former black slaves or Black Loyalists who were listed in “The Book of Negroes.” 
The Black Loyalists on the List of Negroes were taken to British-controlled Nova Scotia where they formed the first free settlements of free Africans outside Africa. Despite British promises of freedom and land, they soon were subjected to racial discrimination and even slavery and to very difficult conditions.
By the 1790’s the Black Loyalists had given up hope of fair treatment in Nova Scotia. They were ready to leave for a new promised land, and soon their opportunity arrived in the form of the Sierra Leone Company, eager to recruit Black Christians for their new colony on the west coast of Africa. Many of the Black Loyalists decided that an uncertain future in Africa was better than certain misery in Nova Scotia.
In January 1792,15 ships with over 1,100 Black Loyalists left Nova Scotia. When they arrived in what is now Sierra Leone in March of that year, they met conditions that were not better than what they had left.
Today the descendants of the Black Loyalists from Nova Scotia are an important ethnic group in Sierra Leone and still meet and dominate certain churches.
As a white U.S. citizen in 2013, I confess that I did not know any of this history until I had read the Lawrence Hill novel about The Book of Negroes and did research for this and the earlier post referencing the novel.
For the African slaves in the colonies in 1775, the Dunmore Proclamation must have seemed like the once-in-a-lifetime opportunity to escape the horrors of slavery. Yet it required great courage for anyone in those circumstances to attempt to, and actually, escape slavery in the hope that they would be free people. I give thanks for their courage and for their descendants’ creation of a Canadian society to honor their ancestors’ courage and history.
At the same time, once must also acknowledge that the estimated 30,000 Black Loyalists were only roughly 10% of the black slaves in the colonies at the time. The other 270,000 black slaves did not have the courage to try to escape or for whatever reasons had decided to cast their lot with the rebelling colonists. Some even fought for the colonists in the War. It would be interesting to know more about them.
1 This post is based upon secondary sources, primarily upon the superb “Black Loyalists: Our History, Our People.” I encourage comments correcting any errors in this post or amplifying on the history of the Black Loyalists.
2 In response the next month (December 1775) the Virginia legislature passed a law that prescribed death for “all negro or other slaves, conspiring to rebel or make insurrection” against their owners while offering pardon to those who ”return in safety to their duty.”
3 “The Book of Negroes” was mentioned in a prior post along with the novel of the same name by Canadian novelist Lawrence Hill. A subsequent post will discuss recent comments about the novel by Hill along with some of his biographical information.