Assuming you've done it by adding a blur filter. Open the clip in the viewer, go to the point where you want the blur to start, click over to the filters tab, click the key frame button to the right of either radius or amount (depending on the type of blur you are using it will be radius in respect of a gausian blur, amount in respect of most others) and use the slider to set it to zero. Scroll forward to the point where you want the "blur in" to be complete, click the keyframe button again and increase the radius / amount slider until you are happy with the result. Job done.