Orca are not whales. Technically, they are really big dolphins. They are in the delphinidae family (dolphins) which explains why they look/act/sound/move so differently than other "whales".
But not all cetaceans are whales. If orca are whales, then so too are the other dolphins. "Whale" is really a lay term covering the really big aquatic mammals. A whale shark certainly isn't a whale, but the term is still used there in the common name because of the association with size. Flipper wasn't a whale.
Yeah, certainly orcas can make some pretty high-pitched squeaks. But I'm guessing they can't interbreed with blue whales.
I guess I find the blue whale + fin whale hypothesis a little odd (why would the hybrid have a call that's higher in frequency than either of the original species'?), but I don't have the expertise to comment on it.
Hybrids don't always get averaged traits. Take my dog, a mutt. She is bigger than both her pure-bred parents, taller by more than an inch. In big cats a tigon, a lion-tiger cross, is bigger than either. The principal even holds true for humans. Obama, of mixed race, is unsurprisingly taller than both parents. This is sometimes called hybrid vigour. My point is that a cross between two different animals doesn't always result in something in the middle. This crossed whale may be very different than either parent.
> In big cats a tigon, a lion-tiger cross, is bigger than either
I don't think this is right. The hybrid with a lion father and tiger mother is the huge one, but the father comes first in the name of the hybrid; tigons (male tiger / female lion) aren't so large.
The reason for the difference is that male lions brand their offspring's DNA with the urge to grow large, while female lions counter with the opposite urge. For a lion/lion pairing, this balances out, but tigers don't have the same war between the sexes going on, and the maternal tiger genetic contribution is defenseless against the paternal lion one.
Humans, by the way, do something similar: the same genetic defect causes "Prader-Willi syndrome" if it came from the father, and "Angelman syndrome" if it came from the mother.
However, these calls are produced via an entirely different vocalization method.