A syllogism printed on a series of placards that I pass every day on my way to work says this: “If you believe that all lives matter, you are right/ all lives should matter/ but all lives can’t matter/ until Black lives matter/ to everyone.” I’ve wondered since I saw it first, what can be said about how Black lives changed in the half-century between the assassination of Dr. Martin Luther King Jr., in 1968, and the murder of George Floyd, in 2020? I wanted a survey. I’ve read several good books about those years, including Fault Lines: A History…