Blurring background to fake depth of field

you need to separate your foreground and background video layers using masks, holdout mattes and the like .... you then apply blur to only the isolated areas of the video in order to fake the depth of field look

FCP is probably not the best application for handling complex ever-moving and ever-changing masked shapes

if you have it, then you would have better luck using Shake for something like this.
if not, then take a look at Motion's masking options instead ... there's a forgivably annoying but instructive video here

do this diligently, don't cut corners or say "good enough". Take a look at the television show Heroes. They do it all the time, and they do it poorly. It seems the editors of the show don't know very much about rotoscoping or keyframing. The blur continually overlaps into the foreground and blurs the characters' faces, but only parts of their faces.

Motion has b-splines. Use them as opposed to bezier masks. B splines are WAY faster when you get good with them.