Have you read the committee report? It's very detailed and well researched and concludes they were likely in blackface, which was very, very common for the time. Given the program and the newspaper accounts they cite, I would argue it's almost indisputable.
But context matters. If you wanted to do an amateur variety show of skits and funny songs back then, at least part of it would have involved blackface, all across much of the US well into the 20th century. Minstrel shows were arguably one of the first true uniquely American artforms - an opera for the people if you will. It was so ingrained in the culture I doubt Sinclair et al. really thought much of it, nor did his audience. It's not like they said, "hey should we make this a normal variety show consistent with 2020 norms or the really racist one?" They likely just did what you did back in those days, on autopilot, much in the same way many of us played "smear the queer" on the playground without reflecting much on its label.
But trying to tie the song itself to where it debuted - likely a coincidence of writing and timing - has always struck me as a bit silly, and it ignores the much more important role it has played historically as a positive and unifying force.