Ok, thank you both.
I was able to but something together:
http://jsfiddle.net/rbL7bf04/7/
It’s not a blur, but a zoom shader. But the idea is the same.
I’ll play a bit more with it latter to make it look supa keoool.
So this is the only way to do that? Passes with masking? Isn’t that heavy, for a mobile target for example?