Without trying to confuse the issue too much, Congress could pass a law that makes slavery legal again, which is to say, that it is allowed by law. The flip side to that though, is that the Constitution would prohibit such an action.

That is clearly of little comfort in the meantime, but it does mean that law enforcement officers are not necessarily bound to enforce the law, courts are not necessarily bound to uphold it, and it likely has a good chance of being overturned by the Supreme Court (if not before).

American Jurisprudence has this to say on the matter:

    It is impossible for both the Constitution and a law to be 
    valid, one must prevail. This is succinctly stated as follows:
    The general rule is that an unconstitutional statute, though having 
    the form and name of law, is in reality no law, but is wholly void, 
    and ineffective for any purpose since unconstitutionality dates from 
    the time of its enactment, and not merely from the date of the 
    decision so branding it an unconstitutional law.

    In legal contemplation, it is as inoperative as if it had never 
    been passed...

    Since an unconstitutional law is void the general principles follow;
    that it imposes no duties, confers no rights, creates no office, bestows 
    no power of authority on anyone, affords no protection, and justifies no 
    acts performed under it.

    A void law cannot be legally consistent with a valid one. An 
    unconstitutional law cannot operate to supersede any existing valid law.

    Indeed, insofar as a statute runs counter to the fundamental law of the 
    land, it is superseded thereby.

    No one is bound to obey an unconstitutional law and no court is bound to 
    enforce it.