BlitzPlus doesn't do native transparancies and such, so you either:
1) Need to read each pixel on the screen, apply the effect you need, and write the modified version back (which is pretty slow! you'll probably only going to get a couple of frames a second at best, not very smooth)
2) use a 3rd party library that can apply transparancy effects.
3) use one of the blitz versions that does 2D-in-3D rather than old fashioned 2D-only. Both Blitz3D and BlitzMax are much better suited for getting the effect for you want here, because they can ofload the work of doing the blending to the 3D processor on your video card.
1) Need to read each pixel on the screen, apply the effect you need, and write the modified version back (which is pretty slow! you'll probably only going to get a couple of frames a second at best, not very smooth)
2) use a 3rd party library that can apply transparancy effects.
3) use one of the blitz versions that does 2D-in-3D rather than old fashioned 2D-only. Both Blitz3D and BlitzMax are much better suited for getting the effect for you want here, because they can ofload the work of doing the blending to the 3D processor on your video card.