Can someone explain Eldritch Blasts to me narratively? I'm assuming Forgotten Realms because it's the default, but if anyone has thoughts on it for other settings, that's welcome too.

What confuses me is that I understand most people in-universe wouldn't necessarily distinguish people based on Class because that's a purely mechanical category. Like people might look at a Barbarian, Fighter, or Fighter2/Rogue1 as just a warrior, soldier, or knight, while they might look at a sorcerer and a wizard and see two mages. However, at what point do mechanics cross over into narrative and vice versa?

What I mean is, Eldritch Blast is uniquely a Warlock spell. Not just a spell, but a central feature of the class (which for some reason is optional), so while an average person wouldn't be able to tell the difference, wouldn't any knowledgeable mage be able to see an Eldritch Blast being cast and instantly be able to tell that "hey, that person must've made a pact with some kind of being" or something like that?

Pretty new to DnD/tabletop so please don't tear me a new one for stupid questions.