YouTube has a tool in its Enhancements area, for videos that you have uploaded, that will allow you to do this easily. You can quickly blur all faces or use the custom blurring tool to select the regions you want to blur. The nice thing about the custom blurring tool is that it will follow the region you select, meaning that if you select a face and that face moves around the screen, it will continue to be blurred. You can even select how long you want that area to be blurred so you can hide a piece of information for a short time and reveal it later, I am sure there are some times this will be helpful.
Watch the video below to see how to get started.