That's not the same as foreshadowing within the same movie by saying, let's inject this magic blood into a dead tribble I just happen to have rotting in the medical bay for no good reason. That just screamed out plot device to pretty much everyone.
That's why the blood was injected into the movie's plot in the sequence of events right after the title card.
The Tribble-scene was just a reminder for the audience, and a welcome distraction for Kirk as he was losing his argument with Khan.
Well, the audience knows about Khan's blood, but McCoy doesn't.