I would like to make all car tags in a given video unrecognizable. I know there are methods of doing this automatically, as it happens on Google Street View.
Is there any tool that could process my video and blur vehicle plates, frame by frame?
There are several methods for doing this. Google has their own custom-written application for doing this... but it's highly customized to suit their needs. There are tools out there like "DeLogo", but the software is designed for statically positioned elements. Recognition software is typically very expensive and tailored to your needs.
Many countries require you to manually blur license plates, as automatic software out there fails frequently.