Simply not true Bliss. While the word Slave or Slavery is never mentioned explicitly in the Constitution, the founding fathers, clearly sanctioned slavery, using terms like those "bound to Service" per below.
http://ashbrook.org/publications/respub-v6n1-boyd/
The first indication of slavery in the Constitution appears in Article I, Section 2. This is the three-fifths clause that explains the apportionment of representation and taxation. It reads:
Representatives and direct Taxes shall be apportioned among the several States which may be included within this Union, according to their respective Numbers, which shall be determined by adding to the whole Number of free Persons, including those bound to Service for a term of years, and excluding Indians not taxed, three-fifths of all other Persons.
Thus this clause explains that the number of persons in each state, for the purpose of representation and taxation, is to be determined by adding to the whole number of free persons three fifths "of all other Persons" The phrase " free Persons" is followed by an explanation that in addition to free citizens, all people bound to service for a term of years, such as an indentured servant, are also included in the total number for representation. Indians, which are not taxed, are excluded from the population to be represented.
"Yet the original U.S. Constitution was
widely thought to have sanctioned this crime. Even today, many still believe that, until the ratification of the Thirteenth Amendment prohibiting involuntary servitude, slavery previously had been constitutional, and for this reason, the original Constitution was deeply flawed.
https://scholarship.law.georgetown.edu/facpub/1238/
"Morality did nag at the consciences of some white Americans—the Enlightenment philosophies of natural rights and growing religious convictions were a nuisance for those profiting from the institution of slavery. The contradiction couldn’t be denied: philosophies that recognized the rights of the individual were juxtaposed against the fact that America had become a place where an entire subset of people were commoditized and dehumanized.
The answer was pretty simple: clarify who gets to be a person and who doesn’t. Fabricating a subservient order for those with darker skin allowed our founding generation (and generations after) to define "all men" and "the people" as "white men." As a result, they guaranteed white men the rights and liberties promised by the Constitution while preserving a thriving economy based on racial oppression."
https://www.montpelier.org/learn/slavery-constitution-lasting-legacy