A tome-warlock built using the SCAG as a rules option could find appropriate EE spells and scribe them into her Tome of Secrets, but a blade-warlock doesn't have that option.
And, since cantrips aren't contained on scrolls, a character won't ever find a cantrip from any rules source that way, thus won't ever be able to add a cantrip from an unallowed rules source (barring some other cert-enabled option that might be created in the future to allow just that).
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Pauper
Edit: And I'm not sure I agree with the assertion that blade-warlocks have to use the SCAG to be viable -- it seems to me that the design intent of the warlock was to have Hex automatically scale as the warlock goes up in level to gain the bonus duration. Granted, combo-ing Hex and Green-flame Blade seems like it might work pretty well, but only for low levels or for blade-warlocks that decide not to bother with the invocation that gives them the equivalent of Extra Attacks. Not sure I'm a good resource for analyzing optimization stuff, though.
Hex works with Eldritch Bolt as well. The issue with Blade locks is that yeah you have a weapon and can even get a second attack... Those attacks will never be as good as Eldritch bolt. Using Eldritch bolt, plus hex will always be better than using your class feature granted weapon... Unless you use green flame Blade which has even a modicum of chance of being on par with Eldritch bolt (Eldritch bolt will still be better for single target damage). Gen flame Blade + hex is almost no matter what better than two attacks, especially since you can use things like great weapon fighting with green flame Blade. Two attacks may take precedent in some situations or for a very short period before the damage of green flame Blade goes up, however if we are getting to higher than level 10 and I want to be a blade lock that actually uses their Blade effectively green flame Blade is a must have.
Also I was less asking about finding a cantrip I was more inquiring of if I could maybe find a scroll of investiture of flame to add to my Blade lock that was built with scag and already had green flame Blade
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