Yep, a "cannot be negated" card cannot be interrupted.
From the REG (the official Redemption Guide)'s definitions:
Cannot be Negated
Cannot be negated limits negates to other abilities. These abilities are unique because the cannot be negated ability cannot be targeted by a negate. Other cannot be abilities can be negated. For example, Job’s Wife can negate Elizabeth Elisabeths protection from capture, and then a player could use Unholy Writ to capture Elizabeth Elisabeth. But Job’s Wifes special ability does not negate Thomas special ability because it is a “cannot be negated” special ability. If a card states that it cannot be negated, then it cannot be interrupted, prevented, redirected, or negated.