Is there any food safety advantage in blanching meat prior to cooking sous-vide?
Pre-searing and post-searing are generally employed for crust formation on the final product.  In fact, when cooking something like a steak, a pre-sear, followed by sous vide, finished with a post-sear allows quicker final crust formation.  This reduces the possibility of over cooking your steak with the final sear. This works with poultry as well.

A quick blanch can be helpful when cooking meats at a low temperature, for a long time. This blanch eliminates lactobacillus, which is not harmful, but could cause off odors and bag inflation. For example when using a low temperature water bath (sous vide) to cook short ribs for, say, 48 hours...or something like oxtail, which I have done for as long as 100 hours, a quick blanch at the beginning drastically reduces any surface bacteria that might incubate and inflate the bag.  Searing will serve the same purpose, but sometimes, with irregular cuts, it is difficult to sear all sides.  Generally, I would suggest a very good sear, or a blanch when using low temperature, very-long duration (12 + hours) cooking situations.

Also, a quick blanch before cooking some vegetables will deactivate browning enzymes, and could be useful.