Blood has nothing to do with it, IMO. It is the GAME that the dogs like. Chickens are the ultimate squeaky toy! Most dogs, once the chicken stops 'playing', the game ends. If it were the blood they craved, they would eat the bird. If they can ever be trusted around the birds again after they have learned how fun the game is, and killed a bird, depends on the individual dog and how good of a trainer the owner is.
My 8yo has a high prey drive, but is very motivated to please me. She has been rather easy to retrain, after her killing a cockeral. During the process I actually could watch her work it out in her head. I was watching out the kitchen window when a pullet ran across the yard. My girl fixated on her, then look away, then fixated on her, then look away. After the third time, she just walked away. I immediately went out, praised her to death and gave her a treat (she is highly food motivated). She now understands that chasing the birds is not allowed, and over time, her self control has improved quite a lot. The ducks were out of their pen for the first time when we walked by with the dogs yesterday,
(they are allowed out all the time, but usually retreat into the pen whenever a leave falls, so are never out when we go past for our walks). The 8yo, look at them, but NOT in a predatory manner. The 10yo, well, we still have some work to do with the ducks, as she looked a little to intently at them. Not quite fixating, but not calm submissive either.
I believe the first step is to teach them to look away, or at you, or look at anything else, in place of fixating.