From the perspective of a guy who actually races more than just a fake car on a TV, blocking if done correctly, and in a minimal fashion is a part of racing, and perfectly fair, but any more than what they allow for in F1 gets pretty dangerous, especially if you are in a noticeably slower car.
I was at a race in Sebring about two years ago trying to catch up to the lead cars in my class (Lotus 7 and another Spitfire), and a Lister Jaguar came blasting past me. He caught the lead two about two corners later, and coming into turn 10, the Lotus pulled a bit of a nasty blocking maneuver on the Lister for God only know why. Long story short, the driver of the Lister locked up the brakes, and slid backwards into the wall at a good pace. Consequently, he was air lifted to the hospital suffering from two broken vertebrae, and a broken femur.
If you're going to block, and you like to pretend this is real world racing be sensible about it. Otherwise, this is just a game, and if you have no plans to translate your "skills" to the real world have at it.