The fact that the clause is not under Article I, Section VIII 's Powers of Congress means the Congress cannot legislate it.
Despite the fact the clause IS under Article V, nothing in it enumerates the ability of the States to legislate it.
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But the fact it was IN the Constitution meant the power had to be somewhere.
This left an ambiguity, and in cases of ambiguity, Congress gets to decide.
After the passage of the Act in 1793, every legal authority was bound to enforce it.
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In a nutshell, the clause was enforceable by all, yet actionable by none.
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Again, my apologies. I really shouldn't try to expound on Constitutional Law before I'm awake. :-)
Since the southern states signed on to the constitution, that means they signed on to the clause that forbade corruption of the blood.
Asserting that a slave is a slave due to his mother being a slave is a form of corruption of the blood.
So the slaves that were born after the constitution should have all been free anyways.
In 1842 Prigg v Pennsylvania the Supreme Court struck down Pennsylvania's Fugitive Slave Law, proving yet again that the Slave Power was in firm control of the Federal Government.
There was some ambiguity in the Supremes' language, which lead several northern states to rewrite their Fugitive Slave Laws forbidding enforcement of Federal law by state authorities.
This ambiguity was removed, and state enforcement of Federal Fugitive Slave Law reestablished by the Compromise of 1850.
The bottom line here is that as of the 1850s, the Federal Government and even most Northern state governments had done everything required to comply with the Constitution's fugitive slave provision.
So the South in general, and South Carolina specifically, had no legitimate claim of "breach of contract".
That's why the claim of "breach of contract" by South Carolina secessionists was just bogus.
Their real reason for secession was the 100% constitutional election of Abraham Lincoln's anti-slavery Republicans, a reason secessionists fully acknowledged.
It means they seceded not constitutionally "for cause", but rather unconstitutionally "at pleasure".