How to Blur in and out

Thanks guys, i have to stick with Ricards's suggestion since the number disappear and reappear again, i still can figure out that keyframe interpolation hold thing , when i try it the blur still fade (maybe i'm missing something). But at last i can finally make it work, have to cut and add 6 mask, still don't know how to begin the blur as they all appear from the beginning of the clip so i have to move around the blur in the open area where they will not be visible and reappear at the exact time. Phew, it was time consuming but it works. Since this is my first time editing thanks for the helpful tips, and i'm still open for more suggestion. 

Richard's suggestion is the fastest and easiest way to do this,

forget about all keyframes if you have simple editing

so just add 2 keyframes as you did at first, the only thing you were missing was to select both of

your keyframes and right click on them, then choose hold from keyframe interpolation

If I got you right :

01. on 00:00:27:15 start the first keyframe and set blur to 0

02. on 00:00:28:15 set the blurriness to 20 (a keyframe will be set automatically)

03. on 00:00:42:54 select your second keyframe, copy and paste it there

04. on 00:00:43:54 set the blurriness to 0 (a keyframe will be automatically set)

for steps 01 and 03 you can set your keyframes on whatever timecode you would like, depending

on the speed of your blur to take place, i suggested 1 second