The only sources I have is this article where they cite some historian and/or geneologist, but other than that have no references. I don't trust the source, but I haven't seen any clear sources against it either other than a few articles that try to dispute the claim.
That article is completely false. The term "black friday" refers to the term "in the black," as in a ledger where if you're losing money you are "in the red," and if you have positive sales you are "in the black" (these are not, by the way, racial terms referring to african descendants or native americans, but to colors of ink used in double entry notekeeping). I've read many, many, many, many books on the slave trade both domestic (in the USA) and international and this article is based on nothing besides the author's desire to convince gullible people to believe something that is wrong. Just google the supposed historian/doctor/genealogist or the supposed institute he works at; they don't exist. This is a silly hoax that is offensive, in that, the reality of what happened to injured or sick enslaved people was far more gruesome. What is worse than being sold on a supposed 'discount slave holiday?' Well, try being thrown overboard to die in the ocean so that the rest of the human property onboard won't get sick and so that slave traders can collect full value for your life on an insurance policy. That's way worse and it happened, most famously in the internationally notorious Zong Incident
This was asked a couple days after you asked, and had a slightly different answer, for those curious: https://www.reddit.com/r/AskHistorians/comments/3ut2rm/what_is_the_origin_of_the_term_black_friday/