Final answer:
David Blight means that America has not achieved its full potential and that there are ongoing challenges and struggles in the nation.
Step-by-step explanation:
When David Blight says America is unfinished, he means that the United States has not achieved its full potential and that there are ongoing challenges and struggles in the nation. Blight suggests that despite progress and achievements, there are still unresolved issues that need to be addressed, such as racial inequality and social injustice.