I often use the linear blur fx on text tracks, but it seems to have a bug .
Even with the "amount" slider set to 0, the text is still somewhat blurred! (the problem is the same with Gaussian and others fx), so you can't totally controll it by keyframes.
It's a bug :-(
It's been reported on this forum that, once enabled, the blur is still on, even after setting it to 0. As I recall, the only "fix" is to turn it off. If you need it again, put the event on a separate track so that nothing else is affected.