The American Revolution affected slavery in many ways, too many ways for a person to summarize in a brief post. If you asked five different historians on this era, you'd likely get 5 different answers with some overlap. I'll do my best to talk about major ways slaves were impacted by the Revolution, however, I want to put a disclaimer out that this is not the easiest question to answer due to how complex it is.
Enslaved Populations During the Colonial Period
Slavery was very much a part of American culture during the period of the American Revolution in all of the American colonies. In July of 1776, slavery was legal in every state. While abolitionist movements were already present in American culture during this period, but change was incredibly slow. In order to illustrate just how deeply slavery was interwoven into American society, let's take a quick look into some examples of how many people were enslaved in the American colonies on the eve of the Revolutionary war. First, it's worth noting that America's population was boobing during the 18th century. States like Pennsylvania went from a population of 25,000 in 1700, to 302,000 in 1775, to 433,611 by 1790 -- that's huge! Similarly, enslavement also went up throughout the century. Maryland, for instance, had an estimated population of 233,000 people in 1776, with around 25% of it's population enslaved, somewhere in the neighborhood of 58,000 people.(1) Virginia had the highest number of enslaved people in the American colonies during this period, with 41% of their population of 657,614 being enslaved. Even "northern" states had people who were enslaved, but at a much smaller number. New Jersey's population in 1784 was 149,000 people, with a slave population of 10,500, or 6.7% of the population. In Massachusetts, about 1.8% of their total population was black, but it is not entirely clear how many were enslaved -- but the number was very low. Across America, it is reasonable to believe that somewhere between 500,000 - 600,000 people were enslaved, however, we do not have a fully accurate number due to conflicting sources on this at the time. It should be noted that America's population in 1775 was 2.8 million people, and that almost 1 in 5 people (between 18 - 20%) of Americans were the legal property of another.
Enslavement During the Revolution
When the American Revolution broke out, different sides reacted differently over the idea of what to do with enslaved people. In the early days of the American Revolution, black Americans, some slave and some free, were part of the Massachusetts militia in 1775. They served bravely alongside their white counterparts at both the skirmishes of Lexington and Concord and again at Bunker Hill. One such soldier, a former slave named Salem Poor, preformed so courageously in battle that 14 officers wrote to the legislature recommending a commendation for him. Because the practice of enslavement was uncommon in Massachusetts, and because some estimates put Boston's black population at around 5-10%, there were not known issues with having black Americans serving alongside white Americans.
When George Washington assumed command of the Massachusetts Militia (relabeled the Continental Army) in 1775, he opposed black enlistments and wrote to the Continental Congress requesting that they no longer be allowed to enlist. However, as time continued, recruitment numbers wavered, and when the British started gathering runaway slaves for military service, he had a change of heart. He specifically, approved the enlistments of free black soldiers into the Army. By the end of the war, black soldiers had made up 10% of the Continental Army. But while Congress and General Washington initially rejected the idea of black soldiers, especially slaves from serving in the Continental Army, the British government saw an opportunity opening up.
Americans knew, early on, that the British could do something rather aggressive and promise freedom to slaves in exchange for military service. That fear became a reality in November 1775, when the British governor of Virginia, Lord Dunmore created a proclamation that altered the early stages of the war. Known as Lord Dunmore's Proclamation, the orders themselves were pretty straightforward. Any male slave who leaves the plantation of a Patriot master and takes up arms with the British, will earn their freedom upon British victory. As can be expected, the prospect of freedom was very appealing and hundreds of slaves left their Virginia plantations at the thought for freedom. Some estimates vary, from 800 slaves to a few thousand, but regardless, it caused panic across the state It created a ton of fear amongst the Virginia elite, eventually resulting in Marshal Law for a period. The measure is believed to have been implemented specifically to cause plantation owners to panic, and largely did its job. However, it caused an issue for the British when enslaved men brought their families with them to British encampments, and now the British had to feed and cloth and take care of many more people than they expected. However, when the war was over, those who remained were not abandoned. The British took these families (roughly 5,000 black Americans) away when they also relocated loyalists who were fleeing, many ended up in Canada.
The First Cases of Emancipation
The debate over emancipation exploded during the American Revolution by patriot leaders who saw a hypocrisy between fighting for freedom while enslaving others. The first place that saw emancipation happened with the creation of Vermont, which happened during the middle of the war. Formerly a part of upstate New York, Vermont's 1777 Constitution was the first place to ban slavery passed the age of 21. This was huge in American history, since this was the first time a political body publicly acknowledged slavery was immoral and took steps to ban it.
The next state to ban slavery was Massachusetts, and it happened in a much more public way since Massachusetts had a huge population and was the center of early Patriot activity. A lawsuit between a slave and his master went before the Massachusetts Supreme Court in 1781 in a case known as Quock Walker v. Jennison. This was a landmark case where Quock Walker sued on the basis that his enslavement violated his rights that were protected by the new constitition of Massachusetts. Justice William Cushing stated that perpetual servitude was unconstitutional and Walker was freed and the jury actually awarded him 50 pounds in damages. His owner appealed the decision, but was struck down. Because of the nature of the lawsuit, the judgement ended up freeing all slaves in the state without any legislation by the state legislature. The state of Massachusetts then brought charges against Jennison on the behalf of Quock, who sued for assault and battery against him while he was enslaved. Jennison was found guilty and fined another 40 shillings.
Massachusetts never officially ended slavery with a law until the 13th Ammendment was passed, the court's decision set the precident that all a slave had to do to earn his freedom was sue. This set up a gradual form of emancipation where practially all slaves in the colony were freed by the 1790s. Ultimately, the American Revolution enabled some slaves to earn their freedom and set up a series of events that eventually led to gradual Emancipation across most states that were north of Maryland. Some, were able to escape their captivity through fighting for the British. But ultimately, the American Revolution left little impacts for the majority of enslaved people who lived in America at this time.
Ronald Hoffman, A Spirit of Dissent: Economics, Politics, and the Revolution in Maryland. Johns Hopkins University Press. 1973. pp 9
George Washington to Major General William Heath, 6/29/1780, The Writings of George Washington, 19:93.