I can on two hands count the quantity of folks whom I've come by who have remarked one way or the other about blackface and who also have any idea of what it is and what it is not. Among the defining qualities of blackface itself is artifice applied to fair skin such that it misrepresentationally exaggerates those African Americans outwardly physical traits that whites summarily deemed as ugly and/or indicative of Black's being an inferior form of human, though plenty of whites construed Blacks as simian more so than human. In short, blackface is artifice applied to define qualities of "otherness" with regard to humanity.
But that's' just the application of the makeup itself, and historical inaccuracy in its application does not make modern manifestations of blackface any less opprobrious. It doesn't because, as is explained in the three documents above, there's so much more to it than just makeup applied to fair skin.
Among folks who understand the multiple manners making blackface "20 kinds of wrong," it's unsurprising and not necessarily unnatural that they'd see soot-covered white guys from days gone by and have come to mind minstrelsy, its blackface characters, and all the baggage that comes with it; moreover, it's fitting that one should find such notions repugnant. Accordingly, it strikes me as reasonable that upon seeing the photo of the miners that someone might say, "That photo, even though it depicts miners covered in coal grime, evokes images and notions of minstrelsy and blackface, and I don't enjoy coming to a restaurant and being beset with imagery that does that."
As for what's offensive, well blackface damn sure is. Who decided that it is? Well, one'd think that each of us has individually come to do that of our own cognition; thus the answer is "everyone individually whereupon such a conclusion is arrived at collectively yet, by now, 2019 concurrently." If one isn't among the folks who've come to that comprehension, one should read the above linked-to documents.
"Not necessarily unnatural"? What does that even mean? Instead of being buried in victimhood and having it drilled into their brains that wealthy, heterosexual, white males are out to screw them, maybe it would do them some good to see that life wasn't always a bed of roses for working-class whites, who never had much hope of going to college where they could become tools of progressive morons who believe that, like Neverland, Utopia can be real.
In the present context, which isn't in any way, shape, or form related to minstrels or race, it isn't fitting or reasonable to find a photo of white coal miners with coal dust on their skin "repugnant." But I do find that suggestion repugnant.
I suppose the same people who decided what obscenity is. They can't define it, but they know it when they see it. That's fine. We had a "national discussion," so to speak, on blackface and came to the conclusion that maybe it wasn't a good idea. Knowing what know now, we can agree to find blackface offensive. I suppose that's progress. That doesn't mean we should toss The Jazz Singer on a pyre. It's part of this nation's history, and it should be preserved as a noteworthy work in cinema.