The way you take in consideration unknown unknowns is not by trying to think of every single possible thing.
It's actually quite the opposite.
You start from what you know and what you control. From that you can assess what you can mess up. The scale, the consequences and likelihood of it.
E.G: if you are using a very potent instable new explosive, you have much more unknown unknowns that if you are manipulating a well well know concentrated acid.
Now starting from here you know what the cost of what you don't know could be, and else can choose measures to mitigate that such as more research, bigger safety coefficient, additional staff, insurance, etc.
When we blame a company for not taking in consideration unknown unknowns, it usually means they just tried to build the very minimal legal requirement to put it on the market and make money. They may have known that it was risky, but they didn't bother.