The darkness spells states the following:
If the point the darkness spreads from is an object you're holding or one that isn't being worn or carried, the darkness comes from and moves with the object.
However, there is no prescription for what happens if the object that the darkness is following is broken (What happens to the darkness spell if it is cast on an object that then breaks?) or destroyed, such as when shadow blade disappears after being let go of by the wielder/caster. What would happen to the darkness spell and its positioning? What would happen if the same object were to reappear in space, as is in this case suggested by the phrasing of the shadow blade spell (emphasis mine):
While the spell lasts, you can use a bonus action to make the sword reappear in your hand.
To me, the most reasonable outcome in the event of the object outright being removed from existence is that the darkness stops being emitted, as the darkness is stated to 'come from' the object. The darkness would then reappear should the same object reappear, provided that the spell hasn't ended (no trigger for which has been activated).