I'm assuming this is happening when you zoom into your city and move the camera quickly or try to follow a person or vehicle. The effect is meant to sort of reflect a sort of 'speed blur.'
You can solve this simply by modifying your tilt-shift.
If it is blurry at all times even with the camera not moving then its a graphical error kind of unique to you and perhaps anyone else using your graphic card & drivers. Try the simple solution (that i'm sure you have) and maxxing out your settings if you think the computer can run it. Otherwise update your graphic card drivers.
Happy simming.