The Imperius Curse can cause a blank look - but doesn’t always.
Someone under the Imperius Curse looking blank doesn’t contradict the books at all - it’s shown to be possible. In the books, there are cases where being Imperiused causes a blank look. When Harry uses the Imperius Curse on Travers and Bogrod, they both end up looking blank.
“They’re Imperiused,’ he added, in response to Hermione and Ron’s confused queries about Travers and Bogrod, who were both now standing there looking blank. ‘I don’t think I did it strongly enough, I don’t know …”
- Harry Potter and the Deathly Hallows, Chapter 26 (Gringotts)
In the movie, Harry also uses Imperio, as he had in the book, and it causes the same blank look.
None see Harry’s hand slip from the cloak, his wand pointed at Bogrod.
The door glides shut. The wind dies. Bogrod blinks.
Very well, Madam Lestrange. If you will follow me.
- Harry Potter and the Deathly Hallows, Part 2
This is likely because the Imperius Curse was performed poorly, but it can cause a blank look. A well-executed Imperius Curse doesn’t seem to have any external evidence, since Dumbledore (a very powerful wizard) was unable to tell that Madame Rosmerta was under it.
Krum and the goblin were likely under less-well-cast Imperius Curses, giving them both an obvious blank look. The goblin looked blank in the book as well as the movie, and Harry certainly wasn’t experienced in casting Unforgivable Curses, so that’s consistent across both. We also know for sure that the spell used on the goblin in the movie was in fact Imperio, since we hear it cast. In the book, Barty Crouch Jr. says he Imperiused Krum, which is still consistent with the spell he used in the movie, if he did it hastily or didn’t cast it well enough where it was unnoticeable. Krum isn’t mentioned as looking blank in the book, but it is possible for him to look that way under the Imperius Curse.