Based on some quick googling, the argument seems centered on Virginia's part in the crafting of the constitution.
I believe part of this argument comes from Patrick Henry at a Virginia conference. Any quotes I've come across where he states anything on slavery, emancipation, and the role of militias have all appeared to be from this book: https://www.amazon.com/History-Virginia-Federal-Convention-1788/dp/1332819753
With Henry in mind, the scholars writing about this say that Henry and people like him are what prompted a re-write of the 2nd Amendment during the ratification process.
Not my area of study, but the majority of it seems based on what I would consider to be logical conclusions based on what letters and written records exist and just the background of the figures involved as well as how slavery was involved in other aspects of the founding of the nation.
But again, that's me doing a quick google search, so I can't claim to know what those advancing this argument are working off of definitively.