The Samsung HW-MS650 soundbar was £299 at Currys PC World on Black Friday, but within a month the price dropped by £49 for at least five days.
The Which? price research covered the six months before and after last year's Black Friday.
Or now now not it is no shock that Amazon leads the pack - the retailer started its Black Friday offers closing week.
Prior to last year's Black Friday event, only six in 10 items were cheaper or the same price on at least one day in the six months.
Black Friday is the time when all the retailers and device-makers roll out their deals, using the allure of discounts to try and get you to spend all your hard-earned cash on stuff. Meanwhile, Amazon put its Echo (2nd Gen) on offer as 39% cheaper on Black Friday, when it had been cheaper on at least 13 occasions before that date.
Which? home products and services chief Natalie Hitchins said: "We have repeatedly shown that deals touted by retailers on Black Friday are not as good as they seem".
The De'Longhi Autentica Cappucino Bean-to-Cup coffee machine was priced at £399 by John Lewis on Black Friday.
But Which? found no evidence that retailers were breaking sales laws.
Walmart, Best Buy, Target and many other stores released Black Friday 2019 ads last week.
There are wide selection of deals and discounts available throughout Amazon's week of Black Friday deals.
Last year, around £1.49bn was spent on United Kingdom online retail sites on Black Friday, according to IMRG, up from £1.39bn a year earlier, with smart home devices among the best sellers. "As such, we offer our customers the best value on the high street all year round, including during the Black Friday period".
The biggest shopping event of the year kicks off on November 29 - so we don't have long to go. Previous year shoppers spent almost three times more in physical shops than on a normal day.