So assuming both fighters are healthy, adult human beings with no bone diseases and no other outside factors are involved.
You aren’t breaking your arm this way.
It may not be completely impossible, as stranger things have happened, but it would literally be a freak accident if it happened.
When blocking a hook punch, the idea is to position your forearm so it intercepts the opponents strike on his arm, not directly on his fist. To get a better idea, we’ll take a more extreme example.
Say someone is swinging a baseball bat at you full force. If you take the full force of this swing, you will be seriously hurt. So what do you do? Well, counterintuitive to human nature, you want to get closer. Very close to 100% of the power on the swing is located on a very small section of the baseball bat about 2/3rds of the way down. If you get past this point you can easily stop a full force swing with your bare hands without receiving any injury.
A hook punch is very similar, except the force of the punch is almost entirely concentrated in the fist. If you block correctly, by getting your block past the fist, you are more likely going to hurt your opponents arm than your own. I will caution that other strikes are a bit of a different story. Kicks have a lot more power than punches, so it’s usually better to just not get kicked if possible. Straight punches still have all the force in the fist, but they are much harder to intercept than hooks and can have equal or greater power. If you take a straight punch directly to your forearm, there is a pretty good chance of a fracture, depending on how and where the strike lands.
To summarize, blocking a hook this way is totally safe, and is more likely to harm your opponent than yourself.
