I think that Grant and King acted with a sense of courage and agency that they felt resulted in better choices for themselves. Each one acted in a manner that was different from how others in similar positions were acting at the time. This makes their actions courageous. Grant could have accepted the life of a traditional slave. He was not going to directly gain anything from his actions for the Continental Army. However, the intangible goodness of freedom was intrinsically good for Grant, and motivated his actions: "When I saw liberty poles and people engaged for support of freedom, I could not but like and be pleased with such a thing." The choice he made came at great cost. As a slave, his recorded action in the service could always be invalidated. As a person of color, even if the Colonists won, there was no direct guarantee that his freedom would be achieved. Essentially, Grant fought for someone else's freedom without a guarantee of his own. This is what makes his actions so admirable, and something that transcends what others were doing in the time period.
Boston King took similar daring agency in trying to carve out his own identity in the world. He escaped, under brutally difficult conditions, and joined the British side in the conflict. Boston King's courage comes from the act of escaping and not being afraid of the risks involved in escape. For slaves like King, there was no direct path or guarantee of escape. There was no road map or protection offered to those who escaped. Escaping to the British side made the most sense for King. However, there were considerable risks in undertaking escape, a reality that he himself noted, as the prospect of possible return into slavey, caused "inexpressible anguish and terror," reminiscing that "For days, we lost our appetite for food and sleep departed from our eyes." To be able to undertake something of this magnitude is courageous and is fundamentally different than many others.
In both cases, I think that each man made a choice in the hopes of making his life better. It is difficult to develop a criteria that faults them for doing so. They did what others did not feel they could do. They undertook risks that made their undertakings even more impressive. From my own point of view, I could only hope to possess the courage and will to action that each possessed.