Check out The Articles of Confederation stupid.
I am well aware of the Articles of Confederation, but you said the United States Constitution, which is a very different thing.
As the US Constitution was the more permanent National Charter and governed subsequently to it's adoption, that document determines the rule of law in the US, and that document implicitly makes slavery legal in the US.
Had the Northern Republicans gotten their way, the original 13th amendment would have made slavery permanent.
Just shows what horrible people they were.