In Volo's Guide to Monsters (pg. 81), the mind blade weapon is described:
Weapon (any sword), rare (requires attunement by a specific individual)
Mind flayers can turn any nonmagical sword into a mind blade. Only one creature can attune to it: either a specific mind flayer or one of its thralls. In the hands of any other creature, the mind blade functions as a normal blade of its kind. In the hands of its intended wielder, the mind blade is a magic weapon that deals an extra 2d6 psychic damage to any target it hits.
I have emphasised "In the hands of any other creature, the mind blade functions as a normal blade" and "In the hands of its intended wielder, the mind blade is a magic weapon".
My question is: would the mind blade count as a magical weapon (or at least, does the slashing or piercing damage it does count as magical damage) for the purposes of overcoming damage resistances in the hands of someone other than its intended wielder?
Ordinarily I would assume yes, since a magic weapon is still a magic weapon even if you haven't attuned to it. But given that it specifically calls out the fact the it's a "normal blade" in the hands of someone it's not intended for, and that it's a "magic weapon" in the hands of its intended wielder, it makes me doubt my default assumption. Also that it's gone out of its way to describe something outside of the usual language of attunement (i.e. it could have said "while attuned" or similar like the descriptions of many other magic items do, but instead it defines an "intended wielder").
Obviously either way the not-intended wielder cannot attune to it or deal psychic damage with it; this is just about it being a magical weapon for the purposes of overcoming damage resistance.