Displaying blank screen won't help you because Even if you display a blank screen, the screen is still consuming power.I guess you only want to use camera feed in your app and you have the control of your android phone as well, so saving battery is one thing that you can manage only with your app though you can't control the most of the apps although there are are couple of things that might come handy.
Try to use marshmallow which automatically helps you to save memory (if you have control of device).
You can disable wifi network and other connectivity services to save battery (look for code online but every code won't work on every version). For wifi ,
WifiManager wifiManager = (WifiManager)this.context.getSystemService(Context.WIFI_SERVICE); wifiManager.setWifiEnabled(status); // pass false to shut it down
- you can create your own customize rom(cyanogen mod is most popular ) which can be modified just with minimal number of required system apps and platform apps so that there will be less number of app to manage and which automatically lead you to less number of system resources requests hence longer battery
Hope this will help.Good luck