The group analysed prices of 83 tech, home and personal products for the course of a year, starting six months before Black Friday 2018, and ending six months afterwards.
Only four products were cheaper on Black Friday than at other times of the year.
Meanwhile, six in 10 products, including speakers, soundbars and kitchen appliances, were cheaper or the same price on at least one day in the six months before Black Friday last year.
In the two week period surrounding the sales event, 74% of prices were cheaper or the same price in the six months following the period.
There was no evidence found that these retailers were breaking the law, though Which? said it is “concerned that shoppers are at risk of being confused by Black Friday offers that might not be as good as they sound”.
The group has urged consumers to not get “carried away by the hype” of Black Friday sales, and to do their own research when it comes to assessing the right deal for them.
Natalie Hitchins, Which? head of home products and services, said: “We have repeatedly shown that ‘deals’ touted by retailers on Black Friday are not as good as they seem.
“Time-limited sales can be a good opportunity to bag a bargain, but don’t fall for the pressure tactics around Black Friday.”
She added: “Our investigation indicates that this popular shopping event is all hype and there are few genuine discounts.”
Examples of tracked deals include the Amazon Echo (2nd Gen), that Which? found on offer at £54.99 (39% off) on Amazon. It was cheaper on at least 13 occasions before Black Friday, however.
Meanwhile a Samsung HW-MS650 soundbar was £299 at Currys PC World on Black Friday, but dropped by £49 within a month for at least five days. It was £279.97 at least 13 times in the six months following Black Friday.
In response to the report, a spokesperson from Amazon said: “The claim from Which? with regard to Echo is false and we have made this clear in our response to them. Amazon.co.uk customers were not able to buy the Echo (2nd gen) device cheaper before Black Friday 2018.”