Three words.
Advanced 2 D.
DBPRO drawing commands are slow. The only one I use is Paste Sprite, the others are not good for serious use in demanding apps.
However, if speed does not matter, just use a transparent colour with the box command. Make sure you are running in 32bit display mode.
This snippet shows you four solutions that draw semi-transparent white. The last two require Matrix1's 32bit RGB function to get the alpha channel.
Do
CLS
a2FillBox 0,0,100,100,0x70FFFFFF
Box 0,100,100,200,0x70FFFFFF,0x70FFFFFF,0x70FFFFFF,0x70FFFFFF
a2FillBox 0,200,100,300,RGB(50,255,255,255)
Box 0,300,100,400,RGB(50,255,255,255),RGB(50,255,255,255),RGB(50,255,255,255),RGB(50,255,255,255)
Loop