“The organizers didn’t have a sense of black history to see right away what this looked like,” she said in a telephone interview. To see it herself as a black Quebecer, she said, was “seeing a representation of what is too often your actual place in society.”

Ms. Nicolas, whose group is pressing the provincial government to investigate systemic racism, said the fact that the organizers did not notice what she called the float’s blatant symbolism, intentional or not, shows that “systemic racism is alive and well in Canada.”

The young men who pushed the float were members of a local high school sports team who had volunteered to do the job. Mr. Brouillet, whose video had been viewed more than 1.8 million times on Facebook through Tuesday afternoon, said he regretted that the athletes had been dragged into the controversy.

In a telephone interview, he said many people in Quebec were “willfully ignorant” of racism in the province. “It was a coincidence that could have been averted,” he said of the float.

Of all of Canada’s provinces and territories, Quebec stands out for its nationalism, tied to a separatist movement and a centuries-long feud with country’s dominant Anglophone community that dates to the Treaty of Paris in 1763, when France ceded what was then called New France to the British. Slavery was a part of the province’s life for 200 years, as it was elsewhere in Canada, affecting both black and indigenous peoples; it was abolished in most of the British Empire, including Canada, in 1833.