The goal of Aikido is "katsu hayabi" or instant victory. ....the instant you touch, you have the attacker's center. Blocking is a movement that stops particular movement in a larger flow of attack. .....
Generally, one should try to avoid blocking and strive for movement which always effects the fundamental alignment of the attacker. That's what makes it "aiki" ......
If you look at what Ushiro Sensei is doing with the movements that appear to be "blocking movements". He is really using the energy of the physical movement to move the mind of the attacker. They call this concept "zero power" in that there is no physical power focused at the point of physical contact. So, to an outsider it would appear to be a block but in reality it didn't "block" anything. Body movement made him safe, the "blocking movement" was a way of connecting rather than stopping the attack. This is "aiki". This is what we are striving for, I think.
There are a variety of Asian Boxing systems (pugilism) as well as a variety of levels within the systems. Some systems like Hung Gar or Shotokan begin and remain quite hard and thus their blocks are like you say; i.e. they stop or even break a limb without reference to controlling the opponent's center.
In the softer systems it is often said, "the hard becomes soft and the soft becomes substantial". Thus in Hsing-I, Pa Kua, Tai Chi and even in the upper levels of Shaolin, blocks do the same as the connection you talk about in Aikido with "zero power". Thus, it is said, "the Tai Chi master can move a mountain with 4 ounces of strength".
Even in high-level pugilism of Kosho-Ryu Kenpo, if uke touches tori or if tori touches uke, uke pays a price; uke loses control of his/her center. Thus we have the idea of sticky hands. Sticky hands only works because you are softly locking up joints with compression, extension, or torque while you allow gravity and friction to "drop weight" upon uke's center through the connection.