Answer: Black Friday was first used to describe the financial crisis of 1869. The stock market catastrophe was set off by gold spectators who tried and failed to corner the gold market, resulting in the market collapsing and stocks plummeting. 

Black Friday, as we know it today, originated in Philadelphia around the 1950s. The police used this term to describe the heavy traffic and the intense crowding of the retail stores that happened the day after Thanksgiving Day. In the 1980s, this term was modified to refer to the time of the year when retailers offer highly promoted sales. 

Black Friday does not have any religious or cultural connotations. It is purely a business marketing concept.

It is therefore permissible to purchase items on sale on Black Friday. However, one should adopt moderation and buy things according to neccessity.

It is narrated from Sayyidina Buraida Radiyallahu Anhu when Nabi Sallallahu Alahi Wasallam enters the market he would say: 

«بِسْمِ اللَّهِ اللَّهُمَّ إِنِّي أَسْأَلُكَ خَيْرَ هَذِهِ السُّوقِ وَخَيْرِ مَا فِيهَا وَأَعُوذُ بِكَ مِنْ شَرِّهَا وَشَرِّ مَا فِيهَا اللَّهُمَّ إِنِّي أَعُوذُ بِكَ أَنْ أُصِيبَ فِيهَا صَفْقَةً خَاسِرَةً» . رَوَاهُ الْبَيْهَقِيُّ فِي الدَّعَوَاتِ الْكَبِير

“In the name of Allah. O Allah, I ask You for the good of this market and the good of what it contains, and I seek refuge in You from its evil and the evil of what it contains. O Allah, I seek refuge in You lest I get a bad bargain in it. [ Baihaqi]

This Dua may also be read when shopping online.