I don't know that it was radical but it was novel, especially in this kind of B-movie which would tend toward a more conventional casting in order to garner the best chance of prevailing. Jim Brown starred in two movies in 68, Al Freeman Jr. had a strarring role as did Sidney Poitier, Richard Pryor, Ossie Davis (playing a effete well educated slave that was in many ways against type) and Woody Strode.

One of the more telling roles though of that year wasn't a starring turn at all. Strode, who was on the show Tarzan and starring in two other films also had a brief supporting role that was particularly interesting in context of this discussion. He was only in the opening scene of Leone's Once Upon a Time In The West. A role that was meant to knock the viewer off balance regarding expectations in a movie that had Henry Fonda playing Frank one of the most brutally malevolent characters in film to that point. Jack Elam was a well known and appreciated character actor, Strode and one other character open the film in a shoot out with "the Man With the Harmonica". Most audiences would have known Strode who played starring roles (both as lead and second lead) and Elam and would have expected their characters to prevail in a gun fight because they should have continued on in the film, given the convention that name talent usually continue until late in the second or into the 3rd act. Essentially the director expects that you will recognize Strode and be surprised by the outcome of the face-off which would mean that on some level audiences were not only able, but expected, to see him as a leading character. You can see the scene here, http://www.imdb.com/video/screenplay/vi1055628313?ref_=tt_pv_vi_aiv_1 (Since it is in the trailer I don't think at this point it would be a spoiler.)

Essentially, black leads weren't the norm but they could be expected, especially in a certain kind of film. An exploitation, comedy, period piece of some sort of social commentary. What made Night of the Living Dead unique would be that it was meant to appeal to suburban teens in a non-threatening way where parents would not think twice about their kids going to see it at the drive-in with their friends. Probably similar to how trans people could play the lead in a art houser or serious adult drama (especially one where we're supposed to learn a lesson) but not the lead in a horror or action film today. Except, of course, if a low budget director wanted to garner some publicity in the trades and among better film critics for his film since the race of the actor was just incidental to the story which would also have given academics a signal that this wasn't your normal teen horror film, which Night Of The Living Dead was not. A sophisticated commentary on race by having a black man play a role that did not have an aspect of it's importance be his or her "otherness" was interesting enough that academics and sophisticated cinefiles would have taken notice in the director as important. For the most part it signaled to audiences, and studio execs, that there was more going on with Romero than your normal drive-in/American International fare presented in these venues.

edit: improved the closing of the last paragraph. 2nd edit: cleaned up some bad grammar and unclear sentences.