Yes, he lost a lot of battles: he was in dissent in Adamson, and the Court eventually adopted the “selective” incorporation of the Bill of Rights rather than the categorical incorporation that Black supported. But Black won the war. Nearly every provision of the Bill of Rights has been incorporated anyway, and in McDonald v. City of Chicago the Court acknowledged that its theory of nominally selective incorporation has moved closer and closer to Black’s.
— The First Amendment. Black is famous (or famously mocked) for saying that “make no law” means “no law.” Of course that’s an oversimplification, but it also conveys an important attitude. And Black knew it was an oversimplication; remember that he was in the dissent in Tinker v. Des Moines.
Overall, Black’s First Amendment theory seems to have been categorical protection with categorical exceptions, and no case-by-case balancing of the reasonableness of restrictions. That pretty closely tracks a lot of modern First Amendment doctrine, and as I discussed in my “historical tests” post, we are the better for it.
— As a proto-originalist/textualist, Black also rejected a number of doctrines that it would have been easy and politically palatable for him to accept — like substantive due process. As with incorporation, you could say that this shows his lack of influence, because he lost those cases. But also as with incorporation, the broader doctrinal trend seems closer to Black’s view than the reverse.
— The areas that made him famous should be enough to qualify him (he should qualify on incorporation alone), but they only scratch the surface. He wrote Gideon v. Wainwright, Conley v. Gibson, Testa v. Katt, Younger v. Harris, Chapman v. California, Youngstown, Marsh v. Alabama, Hughes v. Fetter, Ashe v. Swenson, Burford v. Sun Oil, Griffin v. Prince Edward County, Palmer v. Thompson, and probably dozens of other opinions that are taught every year in law school.
— Of course, he had feet of clay. As a young lawyer, he was a member of the Klan, and as a Justice he wrote the majority opinion in Korematsu. But neither of those things eliminates his overall importance or ability as a Justice.