Hi Steampunkette,
Which Rogue archetype allows you to sneak attack on a spell attack roll? I don't see one in A5E or 5E D&D.
This can be done with Rogue 4/Warlock 8 and these three feats: Shadowdancer, Shadowmancer, and Shadow Assassin. The Shadow Assassin feat gives: "While hidden from a target and in an area of darkness or dim light, you may apply your Sneak Attack damage to an eldritch blast." But the damage done will be less than if you went single-classed Warlock and didn't spend the feats, and you'll need to spend your bonus action each turn to hide again if you want to use it each round.
There are two A5E spells that allow you to make a melee weapon attack with which sneak attack can be used: Air Wave and Altered Strike. There are a few 5E D&D spells that allow you to make a melee weapon attack or ranged weapon attack with which sneak attack can be used: Green-Flame Blade, Booming Blade, Magic Stone, and Shadow Blade. But I don't see any spells like this that are compatible with Eldritch Blast variants.