Psychic Blades are a fairly unique thing. They only exist for your attack, and there is no hard rule that delineates whether your attack ends before the monk's counterattack. They vanish immediately after you hit or miss - I'd rule that immediately implies that there's no time for someone else to make an attack with it and the other rules around them imply a very short, restrictive window for their use.
So a soulblade throws it's phy blade knife at a monk.
A) can the monk use deflect missiles?
B) if the damage is reduced to 0 can the monk throw it back?
Thom Everyman- Midgard One Shots
DMing- The Voyage of the Fallen Star
A) Yes
B) Generally no, but the answer is determined by active player - so on the monk's turn, yes, and on the soulknife's turn, no.
Psychic Blades are a fairly unique thing. They only exist for your attack, and there is no hard rule that delineates whether your attack ends before the monk's counterattack. They vanish immediately after you hit or miss - I'd rule that immediately implies that there's no time for someone else to make an attack with it and the other rules around them imply a very short, restrictive window for their use.
My homebrew subclasses (full list here)
(Artificer) Swordmage | Glasswright | (Barbarian) Path of the Savage Embrace
(Bard) College of Dance | (Fighter) Warlord | Cannoneer
(Monk) Way of the Elements | (Ranger) Blade Dancer
(Rogue) DaggerMaster | Inquisitor | (Sorcerer) Riftwalker | Spellfist
(Warlock) The Swarm