I'm not a gamer so your introduction paragraph is over my head 
Just like you can have two keyboards or two mice connected to your PC, you can use an Arduino. However I doubt very much that you can block certain keystrokes on your keyboard; if it's possible, it will be a functionality of the PC or game, definitely not an Arduino.
As you want your Arduino to act as a HID, you will need to reprogram the 16U2 that provides the USB functionality; there are some articles on the web, search for 'uno as hid'. It will be a lot easier if you buy a Leonardo or Micro; they can act as HIDs from the start.