Godverdomme, wat een zware vraag voor r/AskHistorians! I'll give it a shot.

You are right to be suspicious of the Roetpiet “Sooty Piet” concept as an origin for the Zwarte Piet character as it is often still implemented on your Anglophone journey to understand the Dutch, much like children ultimately find themselves right to be suspicious of their own wintertime gift delivery services on their journeys to adulthood. After all, it is at least strange that someone in a 17th century ‘page boy’ outfit would end up bedecked in not just darkened skin but also an oddly specific and comprehensive array of racialized iconography as they tumbled down a chimney, right? Not only is soot generally not brown, but a chimney would not apply thick bright red lipstick, fasten on gold hoop earrings, provide spotless white gloves, or transform white hair forms into an afro. To be clear, Piet in his ancestral, recent, and contemporary forms as he has continuously evolved over time is and has been an entirely fictional character. I say this not to dismiss the Piet phenomena as a valid source of conflict, rooted alternatively in various anxieties or inflicted pain, but to highlight how deeply revealing this allows him to be about both our past and present. Mark Twain used to commonly say that truth is stranger than fiction because fiction is obliged to make sense, and the Pieten can serve as window into the different kinds of ‘sense’ that race, racial iconography, American cultural hegemony, and childhood memory have made over time.

Importantly, your place as an English speaker in this conversation is going to be particularly fraught in the broader historical context of both anxieties over how wintertime gifts to children are culturally mediated and which models are used to understand race and racism. The jolly heavyset man at the North Pole with a big white beard who may or may not be a retired Greek Bishop from a place that is now in Turkey and who is broadly understood to have many names, among which is Santa Claus, was the result of a deeply self-conscious and explicit project of American nationalism and syncretic nation building. The point was to take elements of diverse immigrant traditions and hybridize them into a single consensus American tradition, replicating the success that similarly self-conscious European nationalists had already enjoyed on smaller scales flattening more local traditions into national phenomena. As Dutch speaking children get increasingly exposed to Santa, and gain fluency in English at younger ages, the powerful syncretism of Santa poses an oddly imperialistic threat to the ‘pure’ nature of Sinterklaas, and prompts questions from children that feel new even if they are as old as the diverse traditions themselves. Thus, in a critical way, the Zwarte Piet part of the Sinterklaas traditions doesn't have a single origin story. Like with all fundamentally syncretic cultural phenomena, the Zwarte Pieten are remade anew every time their story is told or shown, and the best we can really do is describe a flow rather than even a linear progression as the wikipedia article does an excellent job of. As it notes, the modern phenomenon of the Zwarte Piet first began to be formulated in the 1850 book by the school teacher Jan Schenkman Sint Nikolaas en zijn Knecht, and was later further modified by both older traditions and new ones.

You have asked whether Zwarte Piet is a form of blackface, and the answer is perhaps complex, but unambiguously yes. Blackface is an anglophone word that describes a deeply American practice, which exploded onto the world stage with minstrelsy in the 1840s. Minstrelsy was notably perhaps the United State’s first major cultural export with American and later British troops of minstrels regularly touring both the Netherlands and Flanders. Indeed, British minstrels still toured the UK, Belgium, and the Netherlands with many of the same full-throated-ly racist narratives and songs into the late 80s and early 90s, with Dutch speaking audiences featuring prominently among the very last places that minstrels could perform. The racial iconography of minstrel blackface may have been influenced by much older Spanish and Dutch imaginings of ‘Moors’ and Barbary pirates, however it also unambiguously directly guided the Zwarte Piet aesthetic. Even the traditional method for blacking up as a Zwarte Piet before industrially manufactured pigment creams could be bought off a shipping container in a store used the very specific and nonintuitive process that minstrels used, which is perhaps best left to fade from our cultural memory.

By transforming the physical features of sub-Saharan Africans into an exaggerated caricature of a costume, which can be tried on as an object of play and taken off, both forms of blackface act to reinforce notions of white supremacy. Zwarte Piet’s role as the introduction that most Dutch speaking children get to the iconography of racial caricature, dominance, and terror might be incidental to his cherished place in childhood memory, but it is undeniably there. In both Flanders and the Netherlands there has for some time been an annual fraught conversation about the Pieten and their future, which has been largely guided by Dutch speaking parents of non-white children who have rallied around the slogan ‘Zwarte Piet is Racisme’ having seen the impact of the racial caricature and childrens’ understandings of it. The conversation has been moving in recent years towards a consensus around either the Rainbow Piet or more generally the Roetpiet, preserving the tradition while stripping it of the racial iconography, perhaps out of fatigue for the annual reformist/reactionary cycle more than anything else.