The "muddiest point" in any work is where the greatest amount of questions or confusion arise. Since it is based on an individual's perception, the best I can do is to offer potential areas of complexity in Zinn's first chapter.
One "muddiest point" might be in Zinn's retelling of Columbus. For so long, Columbus had been seen as heroic. His vaulted position had been a part of the traditional historical narrative. However, it might be "muddy" to have to reconfigure his position in the face of so much evidence. A "muddy" element in Zinn's treatment of Columbus is how someone who did so many bad things to so many people could be seen as glorious by so many. Analyzing this disconnect between historical reality and historical mythologizing could be one of the "muddiest" points in chapter one. The lack of simple and concrete answers makes this a very difficult process.
Another point that might be "the muddiest" could be when Zinn submits his thesis. The purpose of the book is outlined in its first chapter. In the midst of his analysis on Columbus, Zinn puts forth his methodology of how he interprets history:
My viewpoint, in telling the history of the United States, is different: that we must not accept the memory of states as our own. Nations are not communities and never have been, The history of any country, presented as the history of a family, conceals fierce conflicts of interest (sometimes exploding, most often repressed) between conquerors and conquered, masters and slaves, capitalists and workers, dominators and dominated in race and sex. And in such a world of conflict, a world of victims and executioners, it is the job of thinking people, as Albert Camus suggested, not to be on the side of the executioners.
This could be seen as a "muddy point" because we have to consider other motivations in the retelling of history. Readers have to see the difference between the "history of people" and the "history of nations." This can be seen as "muddy."
There is a tendency to see history as "objective truth." Zinn believes there is no such thing as pure objectivity. Seen and unseen biases infect our telling of history. Analyzing these presuppositions reveals political implications. The way we teach and learn history reflects these understandings, a revelation that might be "muddy." Tangentially, it might be challenging for us to figure out when we have been "on the side of executioners." Zinn forces us to reevaluate our own positions. There might have been instances where we clearly believed something and Zinn is asking us to dissect the underlying ideas behind such convictions. Doing this can be a "muddy" and challenging exercise.
In the final analysis, the "muddiest point" of chapter one is dependent on the reader. It is reflective of what they feel and understand as they interpret the ideas that Zinn puts forth. I think two areas where this process could start would be in Zinn's analysis of Columbus and his thesis regarding his construction of the historical narrative.