Whitewolf,
For simplicity's sake, we'll assume the knife wielder is right handed and slashing with a reverse-grip towards me at my six to eight o'clock angles of defense. Try this:
1) Downward forearm block w/simultaneous forearm strike/hammerfist to neck region,
2) maintain forearm contact as you move to your left, use right hand on neck/shoulder of assailant for control (I like to add a knee strike to the midsection here, but not necessary),
3) let forearm slide up to the inside of his elbow as you continue to move outside and use a rear arm bar hammerlock to control or take opponent down (I like to add knee strike to head or footstomp here

).
You can use this against the saber grip stab or slash from this position, also. Nice thing about the reverse grip; you might end up impaling him with his own knife on the blocking portion. While doing this in class, we end up with a lot of (unintentional!) impact disarms.
I hope this helps!