If you want to make something approaching a legal argument, it's the slave owners who are first entitled to reparations. The Fifth Amendment to the Constitution - which existed during the time in question - asserts that citizens may not be deprived of "life, liberty or property" without due process of law. Further, if you are deprived of property (e.g., chattel slaves) by the Government, you are entitled to be compensated for the value of that property.
While the Thirteenth Amendment formally freed the slaves (the Emancipation Proclamation was nothing but propaganda), that doesn't extinguish the obligation to pay for the confiscated (freed) property.
As for what might be owed to the slaves (and their progeny), the source of those funds would have to be the slave owners - or t heir descendants. There is no tenable argument that supports a general obligation of even the Confederacy, let alone the United States, to compensate the slaves (or their progeny) for the value of services rendered.
What is going on in San Francisco is idiocy raised to the x power. California was not a slave state. What would be the source of those funds? If they were serious about the reparations think they would have to establish a rational way of applying for those reparations, and a requirement that the applicant prove descendancy from slaves. Good luck with that. How many Black people in San Francisco have no "white" blood? How many have roots other than American slaves? Who the fuck knows?