I've got a degree in zoology and know a little about
ornithology. The answer to your question probably lies in behavioural ecology and economical use of energy: the blackbirds have a great stake in protecting their nesting territory and so are willing to use the energy required to defend it; the hawk can justify the energy expenditure to try to get at the nest but fighting back and killing the blackbirds to get at the nest would probably use up more energy than would be gained from eating the chicks. It is pretty amazing how animals maximise their energy use and consumption. Birds seem to know exactly how far to go to forage or mate without wasting energy.
And I am sure that the hawk could easily take out the blackbirds if they wanted to. Birds of prey can do some amazing things in the air. Two days ago I saw a demonstration at Taronga Zoo of a kite dipping, diving, and turning on a dime in the air to catch food with its talons and throw it up to its mouth in flight.
Check out my friend's website if you like birdwatching:
http://geobirds.com/