@JeremyECrawford Is the attack made as part of Booming Blade magical? It's delivered as part of a spell, but has its "usual effects" and is a prerequisite for the spell to work.
— Armando Doval (@armando_doval) November 8, 2017
The booming blade spell isn't intended to make the required weapon attack magical. #DnD https://t.co/kFDRhuU8nj
— Jeremy Crawford (@JeremyECrawford) November 8, 2017
What about the automatic extra damage at higher levels? You deal not only weapon damage on hit, but also bonus magical damage created by the spell.
As a DM, I’d personally rule that the weapon damage can be resisted or negated (assuming a mundane weapon), but that the extra damage created by the spell itself obvious counts as magical.
JC’s comment didn’t address the magical damage linked to the spell, but about the nature of the weapon used (non-magical, in this case) and how BB and similar don’t make the weapon’s attack/damage magical, in-and-of-itself. So his response stands as-written. (The bonus damage is just that — bonus damage, linked to the spell, not the weapon directly.)