I'll have to disagree. I can imagine that some set of circumstances exists where Portent would be required to affect a character's mind or sense a character's thoughts in order to function. Forcing a character to take a die on an unaided Intelligence (Knowledge) check, for example, would probably be blocked.
Mind blank says it, "foils wish spells [...] or effects of similar power used to affect the target's mind[.]"
If the effect is at least partially external, however, then I'd say Portent works just fine. Even a Wis, Int, or Cha save might is possible to affect with Portent, since the ability could manipulate the source of the effect instead of the recipient.