For me - a woman coming from Israel - when adding "black" as an adjective to a day in the week - it is usually in a negative connotation.
For example - the "Black Saturday" on June 29th 1946, is known as the day where the British - who ruled Palestine back in the 20's-40's of the 20th Century - put a curfew on the Jewish settlements, and arrested the leaders of the underground mvements, who acted illegally - striving for the establishment of the State of Israel.
Another known black day was the "Black Sunday" - on November 8th, 1942 - where the Nazis slaughtered the Jews of Staszów.
But not here in the USA. Here - the "black" adjective is added in a positive context. Black Friday - is the Friday after Thanksgiving - the day where millions of Americans begin their shopping for Christmass, and the day where the financial books are turing from "Red" (the color for "loss") to "Black" (the color for "revenue").
I would call it - Green Friday. Don' know how it shows in the official financial books. I got to spend a lot of greens today :-)