the black borders are created by your display card, not the game itself, as a result of when the game's screen size is smaller than the display card requires.
There are several ways around this, but neither option is perfect.
1) you can set your game screensize to the same aspect ratio as your current windows display.
Unfortunately due to the max limit of 640x480 on VXA this means that to help a widescreen you would have to reduce your screen height, and it will only work for people who have the same ratio (black borders again for all others)
2) you can use a zoom script adapting windows methods to change the display screen size (NOT the game screen size) beyond 640x480. This will result in some distortions or blurring most of the times, and might not be compatible with the FPLE script.
And that's it - the graphic library of Ace is closed source, reverse engineering it is illegal.
RMMV has an open graphic library but I think there is no equivalent to FPLE yet, although some people started working on something like it a while ago.