They were slaves. They weren't part of the processes that led to the founding of the republic. In other words, that's like saying pack mules founded the USA.
People aren't mules. People have culture in a way that mules do not, and African culture became an important part of American culture and identify in a way that is not true of, say, Finland.
You're comparing black people to mules? Seriously? They contributed their work not to mention what people from US did to native americans.