give it a try ...
aminet - borderblank
in short, it diasables the backgroundcolor outside of the screenarea (compared with the c64 - bordercolor).
it means in a 640x512 screenmode, youŽll see only these 640x512 and not the complete overscan 720x576 in backgroundcolor.
btw. it looks muh more professinal .. like a graphiccard-screenmode with blackborder ...
you can also try to boot from this
all aviable amiga screenmodes are included (with hgfx and hd720) and borderblank is activated.