Selectively blur many but not all of the faces in a video
While its technically possible to have a program detect specific faces and apply a blur on that face. That would require a lot of programming work on your side, there is no free tool or low cost tool that does this automatically for you. This is possible with OpenCV but I highly doubt that this is a viable solution for you in any way, it's a quite complicated task and the result will not be 100% reliable.

The general workflow of blurring out faces would be to semi automatically track the motion of the faces with a 2D tracker in each clip an apply a blur mask to them.
The usual go to software for that is Adobe After Effects or other Motion Graphics video applications like Apple Motion.

Both are no free applications and I never heared of free software that implents 2D tracking together with a video editor. Though there are some really cheap amateur video editing suites out there that even implent face recognition but they all work pretty bad and unreliable without giving you much or no control over whats happening.

For example: http://www.wondershare.com/video-editing-tips/change-face.html
I highly discourage using such software for any work that goes beyond your average random youtube video.

While Adobe After Effects was always unaffordable for hobbyists, Adobe now switched to a subscription based model which makes it very affordable for small projects. You can use the program for free for 30 days (no commercial use) or a monthly subscription for ~30$ depending on the country you live in.

I recommend After Effects because there are ton of beginner tutorials out there for exactly what you want to do and the tracking workflow is quite straight forward. There is also Apple Motion like already mentioned and Nuke and quite a few other applications though After Effects is available for both Windows and Mac and is the easiest to grasp in my opinion.

Edit: I just found out that there is also an automatic face tracker in Adobe Premiere since CS6, this might not yield perfect results but should ease the workflow quite a bit.