In fcpx, Can I edit source media in time line and have the changes be reflected in the project?
Once you have placed a clip in a project, it "breaks" the relationship with the original clip. I suppose this is so you don't make a change to a source clip and inadvertently change it in a project where you didn't intend to change it. I'm not sure that it's useful like that, but its the way it is. 

There is an exception, multicam clips. If you make a change inside a multicam clip, the change will carry through all of the edits. But that won't help you now.

If your blur is in the same spot for the entire clip, there are a few easy ways to fix it. 


Blur your text in one of the clips. Copy the clip. Select all of the rest of the clips. Command+Alt+V to paste attributes, and the blur will be applied to all of the clips at once. 
Download Alex4d's Adjustment Layer title effect. Connect the title clip to your project so that it covers every clip that you want to apply the blur to. Now make your blur. This is nice because you can tweak the effect without having to remove the effect from all the clips and reapplying the clip.
The long way. If you were going to use the exact same setup and want to blur out the same text in the same spot across projects, create an effect with just the blur in Motion and publish it to FCPX. Probably not helpful in this particular situation unless you will use this a lot.