i have seen a professional do decalling quite a few times and what he simply does is to paint all edges of the machine black BEFORE he puts the decals on.
That way, when he uses a scalpal after the decals are done, and he trims the edges using a large metal straight edge, the black finished edge just appears from underneath.

If anything needs a touchup, he uses a very large black permanent marker (texta) which has a very large square head.

The trick is to do the black edge work BEFORE you apply the decals. Hope it helps.