The Spell Sniper feat has absolutely no effect on shadow blade
Let’s go through the effects of the feat:
- Spells that require you to make an attack roll have their range doubled.
The range of shadow blade is “Self”, so nothing changes.
- Your ranged spell attacks ignore cover.
The shadow blade spell involves melee spell attacks, so nothing changes.
- You learn a cantrip.
The shadow blade spell it not a cantrip, so nothing changes.
That said, if, somehow, shadow blade is the only spell you can cast, then it does count towards the prerequisite for the feat, which is having the ability to cast at least one spell. If you can cast shadow blade, a spell, then you can take the Spell Sniper feat.