I edit on Avid Newscutter at work and in order to add black within a timeline, you can simply hit alt+h (it's a PC based system) and it will add a second of black, or if you set in and out points within the timeline, it will add however much black is between the in and out. I find that I use it a lot to create space in a timeline to extend and trim clips quickly. 

Why doesn't this function exist with AXP? It's cumbersome to have to go into the menu and use the load filler command, especially when doing this frequently. This was one of many reasons I preferred editing on Avid as opposed to FCP.

Just wondering if anyone can shed light on this.

You can map "Load Filler" to your keyboard.

Sounds like a neat feature request for Avid's other editors, though.