My issue with shadowblade as it stands is the spell can be combined with another spell of equal or greater power that DOES require concentration that by itself is extremely powerful, coupled with the stone its game breaking.
Shadowblade with concentration would require risk vs. reward using the spell would still be powerful just not overwhelming so.
The duration would still last until LR just with the concentration component, any character using this spell should have a way of protecting their concentration, have a conventional weapon as backup if concentration drops, or just cast the spell again as a bonus action on the next turn.
My suggestion of necrotic was purely so it wouldn't get a benefit from the resonance stones psychic vulnerability, it could just as easily be any other damage type force damage could work just call the spell forceblade.
