A lot of places raise the price so the discount seems good, when a lot of times you're paying the normal price. Amazon is notorious for this. Let's say something usually costs 100 whatever. They'll raise the price to 120 and discount it by 20 so you end up paying 100 anyway. But to people who don't know better it seems like a decent deal.

Websites like CamelCamelCamel give Amazon price histories so you can see if they've raised prices for "sales."

Edit: An extra tip, for places you can't get a price history it's a good idea to make note of prices of things you're interested in leading up to Black Friday. Start a month or two beforehand. This way you can see if the price has been raised compared to what you saw previously.

Edit 2: A lot of companies ≠ every company. So yes, there still are deals to be found out there. It still doesn't hurt to prepare yourself beforehand so you don't get taken.