Question

Why does douglass believe that the continued existence of slavery tarnishes the principles expressed in the declaration of independence?

Answer 1

Douglass believes that the continued existence of slavery tarnishes the principles - expressed in the Declaration of Independence because Douglass does not find it right to celebrate independence in the country there has slavery.
Frederick Douglass was a fiery orator and he is well-known speeches is "The Meaning of July Forth For The Negro.

Answer 2

Just beneath the saccharine lore of celebration that currently occasions our nation’s birthday on July 4th is a rich, luminous underside of civic verbiage that explains the pangs, possibilities and challenges that have shaped the contours of American democracy.

Independence Day in recent decades has unfortunately become the symbolic gateway to summer in America — featuring grilling and grog, picnics and pyrotechnics, grandiose commercial send-ups of summer cinema, baseball and buoyant, unending hours of beach time.

But the holiday celebration was originally a matter of serious social engagement, emanating from the nourishing political environment of civic life, which then gave birth to highly original forms of constitutional governance, new polity norms and public discourse. Fostered by the American colonist’s economic and religious grievances that led to war with a brutish British empire, July 4th was the sacred civic product signifying the birth of a new nation.