Blocking is one of many techniques in this game.
At least in a 4-player game, similar to "just grabbing all the 6-point routes", you are unlikely to win by adopting a strategy of primarily blocking. But it's a tool in the toolbox.
I've tried many strategies in my years of playing this game. If you go to a "primary blocking" strategy you can manage to take down one person, but you're likely to just be surpassed by the other two. You can't block everyone, unless you get very lucky and everyone depends on one critical city you've blocked off.
That said, I use blocking as a tool occasionally. If you've telegraphed your route, and especially if I can get where I'm going by taking what you clearly want, I might as well do so. This goes magnitudes more if you've left a critical segment open turn after turn while you go on and do other things.
There are certain choke points on the board, but most of the time there are easy ways around. In the few times I've tried to block someone from getting a coast to coast route, they have quickly gone around and I just waste time and train cars.
If I need to get to Miami, I will often try to take the two longer segments, as that's 25 points, plus it makes it harder for others to get in. On very rare cases where I have little else to do I might grab the 4-car pink segment as well. But how do I even know anyone WANTS to go to Miami?
Nobody is trying to make other people have a bad game. They're trying to dominate the others and have a good game themselves.
