There is no doubt that knifes, shanks etc. are really very dangerous. Vegetius wrote in De Re Militari:
[...] a stab, though it penetrates but two inches, is generally fatal.
The Fairbairn Manual of Knife Fighting says
[...] we state there is no means by which an unarmed man can defend against a knife fighter. Still, it must be acknowledged that there may come unfortunate circumstances in which one has no other choice but to make an attempt.
I heard that the Russians taught one arm raising block (age uke) to their soldiers in Afghanistan due to the common overhead ice pick attacks. I don't know how it worked out for them.
Using Juji Uke to trap the arm holding a knife seems like a bad idea. The attacker can collaps his arm and draw the blade from head to toe or plunge it into your collar. You could however use one forearm as a buffer and one hand to grip his wrist. Like in this video. That might work.
What are your thoughts on this?