FWIW, the Heirloom thing isn't a 5e thing, it's just something I've been doing in my games since forever now. Foci in 5e don't really have mechanical weight, but as an Heirloom item, it absolutely could.
Perhaps for the +1 feature your Spell Save DC/Spell Attack Modifier is +1 for any spell you can cast using the Foci (so it would work on pretty much all spells with material or somatic components) then for any spell that could be considered Shadow Magic, you'd get + your proficiency bonus as the same sort of modifier. For our purposes, I'd be willing to call pretty much any Necromancy or Illusion spell 'Shadow Magic' and would be willing to look at specific spells from other schools that you think might fit on a case by case basis.
That's just one direction you might go with it. I'm happy to entertain other approaches.