The advantage should be less than the attacker being invisible. The defender would still be able to see the attacker's arms, so would have some idea of the timing and placement of the attack.
It certainly would help to set up a surprise attack, but no different than an unarmed attack on surprise round.
For the feint; feint is more than just fancy weapon tricks. It's about confusing the defender sufficiently that they are exposed. Not knowing where the blade comes from certainly would help.