How do I blur a face?

Hi. I am going crazy. I created a shape, and then I added the blur effect into the shape. It doesn’t work. I have to choose a colour! I don’t want a colour! I just want the area to be blurred. Also, I can’t change the parameters of the blur! Where are they?

Apply the Blur effect to the clip. The effect can be masked with a shape mask at the top of the effect controls.

Oh! So, first I apply the blur to the whole clip? An then try to mask the effect off?

That’s the simplest way.

Does FCP provide motion tracking on the blur?

There is no automated motion tracking. You can manually keyframe the effect, or there are third party plugins like TrackX from CoreMelt.

I should probably add that tracked blurring is in SliceX not TrackX. TrackX is good for screen replacements, and DriveX is for 3D and callouts, but SliceX is for affecting an area of your shot (blur, color, etc.).

I need more help. No masking occures. What blend mode should I choose in Compositing? There is no differrence whatever I choose!
There is also a view parameter with 2 options. Original or composite. In composite everything outside the mask is black. In original nothing happens. 
I have added both the blur and the mask to the clip.

This is a tutorial video showing one way to do exactly this:

Let's step back, I don't think you need to mess with Composite modes. If the video above helps, great. Otherwise, there are a few ways to do this.

1. Add a Blur effect, add a Shape Mask to the blur to limit the area of effect, then animate the position of the shape mask to cover the face as it moves.
2. Use a plug-in like SliceX to blur the face, then automatically track the position of the mask.
3. Use an effect like Censor (which can blur or pixelate) and then animate the center of the effect. This last method is shown in my book (fcpefficientediting.com) and the relevant page is attached.