Well, if you have a separate budget for every purchase/investment forms then you shouldn't have a problem with buying yourself something on a great discount - that's how I organize my finances. If you don't, then you have to be at least disciplined not to go overboard with spending money you could've used to invest (of course, spending Bitcoin directly is not wanted at all).
It all depends on how organized are you personally, it will reflect on the investments you make.

As for Black Friday, I've seen some good deals and I have made some purchases but only on stuff I already intended buying and I knew the price before so I made sure it's really a discount and not a marketing trick.
You can genuinely find some good discounts but there are also many fake discounts you should watch out for. My strategy is following the price of my wishlist items so I know if a discount is real.