MarkB
Legend
Given the age of this thread, I'm guessing it also does necrotic damage?I think it's a cool idea, but like you said--the rules don't really allow for it. If it's important to your character's build, why not talk to your DM about making it possible via a new Eldritch Invocation? Maybe run this one past your DM and see if they allow it:
Gloom StrikePrerequisites: Agonizing Blast, Sneak Attack class featureYour eldritch blast cantrip is considered a ranged weapon for the purpose of your Sneak Attack ability (only).