How to keep the screen from going blank or locking for particular application
It's odd that it happens. My last few devices always stayed on while Navigating, when Navigation was a stand alone app and now that it is part of Google Maps. You could go to Settings > Display and turn the screen timout off. Double check the settings in Navigation. 

Alternatively  Screen timout toggle  is an application on the Play Store that seems to be what you are looking for.