Yea, I guess I don't get it either. I know someone who can eat at a local restaurant for free whenever he wants because he knows the owner. In return, he helps the owner maintain his car and does little odd handyman jobs around the owner's house for him. Is this blat? Is it corruption? Or is it just friends doing each other favors?
From the sound of it (I have never heard of blat before this post), the important distinction is that the owner is on board with it. If he could eat for free because he knew a server who would give him the employee discount, it would be blat. If he worked as a mechanic and took parts from his employer to repair his friend's car, it would be blat.
It's a slippery slope.
What rules are you breaking to do your favor? What rules do you expect someone else to break for the return favor? What rules might they later expect you to break? To what extent do you stop seeing the rules of external society as rules that you're supposed to follow?
It starts as favors.
By the time you're stealing from your employer, it's blat.
By the time you're recruiting one friend to submit paperwork to help another friend commit insurance fraud, it's still blat. But also its starting to look like something else.
Once you owe a favor to a Mafia Don, it's called organized crime. But the underlying blat is still recognizable.
It's pretty tiring seeing so many people push the bounds of acceptable behavior. It's pretty simple: should someone in your chain of management discipline you for setting aside that cheese? If yes, you are engaging in corruption.
That action is basically stochastic theft from the grocery store, because you've altered the pricing of a possibly scarce good.
We call that restaurant thing "mate's rates" here. There is a symbiotic relationship there, a trace of barter and also keeps work off the tax books.