Three things wrong here. The Constitution was drafted in 1787, not 1781, and secondly the Constitution prohibits congress from passing a law to abolish the slave trade until 1808.
If the constitution specifically bars congress from passing a law to stop the slave trade until 1808, it demonstrates that slavery was inherently legal on the national level.
Thirdly, Article IV, Section 2 is the "fugitive slave clause" of the US Constitution. It specifies that slaves must be returned to their masters.
So yeah, the US Constitution has slavery incorporated right into it, though they go to great lengths in avoiding the usage of the word "slave."
You probably believe the United States invented slavery, dont you?
Check out The Articles of Confederation stupid.
Nice summary. Though you would have included Article I Sec 2 para 3 “three fifths of all others”