If a law was written that said blacks are not "person" for the purpose of making slavery legal again...
Now you are talking.
You are giving the "purpose".
However, the Constitution is not about the definition of an individual as a person but about human rights. In this case, the rights of the black person.
Every time the Constitution is used, the issue in question has been rights for marriage, rights for life, rights for carrying arms, rights, rights, rights..
(To define "person" you must consult the US Code Title 1. GENERAL PROVISIONS Chapter 1. RULES OF CONSTRUCTION Section 8. “Person”, “human being”, “child”, and “individual” as including born-alive infant:
(a)
In determining the meaning of any Act of Congress, or of any ruling, regulation, or interpretation of the various administrative bureaus and agencies of the United States, the words “person”, “human being”, “child”, and “individual”, shall include every infant member of the species homo sapiens who is born alive at any stage of development...)
(I don't agree with such a definition but such is what the Act of Congress has stipulated)
In the past, blacks were always considered as persons, only "three fifths of a person" but persons at last. The reason for this condition was to avoid their right of vote.
Then, you have a definition -
of three fifths of a person - which
was valid in those times to take away some rights from a group of the population.
However,
it was established that slavery was to be ended in a certain number of years after the writing of the Constitution.
Your hypothetical question can't be answered if you are not establishing first what is the position of the Constitution in your example about slavery, slavery rights, slavery trade, etc.
But, if you use your hypothetical question applying the current Constitutional determination, then you have:
The 13th Amendment. Section 1:
Neither slavery nor involuntary servitude, except as a punishment for crime whereof the party shall have been duly convicted, shall exist within the United States, or any place subject to their jurisdiction.
There you finally have the answer to your question, an answer based on the 13th Amendment, where no matter if blacks are or are not considered "persons":
slavery is not constitutional.
Then, the Supreme Court doesn't have to make deliberations about the law of blacks as persons as being constitutional or not for the purpose of slavery, simply because slavery is not allowed in the United States.
In my opinion, the Supreme Court might be called to intervene and decide or give an opinion on the internal conflict in Congress between the Act of Congress Section 8 and the new law of blacks as "no persons".
If the Supreme Court -of your example- refuses to take sides on the issue -from above-discussed in Congress, then, can you please change your question? I don't know sh*t about legal stuff...