Personally, the blanching in boiling water makes no sense.
People suggest thus as a method of stopping loss due to lactobascillus bloom which is given as the reason for loss of some long cooks. However… Lactobacillus strains are rendered inactive at temperatures above 120°F and will be dead long before they can cause a bloom that will damage the meat.
The greyish green that is sometimes in evidence on the surface of a long cook is caused by myoglobin, not by bacteria. Wipe the meat dry before bagging for cooking and you are less likely to witness this phenomenon.
Bacterial bloom is also given as the cause of loss of meat during a long cook. The meat when released from the bag has an ‘off’ smell. Once again, the cause of this is not bacterial. The bacteria is rendered inactive long before it can create enough of a bloom to ‘turn’ the meat. What is actually happening is loss of product due to autolysis. This is the same natural process we make use of when aging meat. Enzymes contained within the meat commence the breaking down the meat. This process starts when the animal is dead. It is slowed by refrigeration, but not halted. At ambient temperatures, even in a 100% sterile environment, meat will ‘go off.’ At the temperatures used for sous vide cooking the process is sped up. Cooking low temps and long times is a balancing act to get the temperature low enough to produce desired results while cooking fast enough to beat the natural process of autolysis.
Autolysis is also the reason why pasteurised food will not last indefinitely under refrigeration. Pasteurisation and refrigeration have rendered bacteria within the product dead or inactive but the product can still go off over time. Autolysis is the reason why.