OK, well obviously the immediate answer to that is it's what Roald Dahl chose, but why? If he'd decided to turn her into a tomato, would we all be thinking about that? I have to say i have something of a weakness for tomato costumes, and it could have gone that way for me. The gum turned into tomato soup before it became blueberry pie.
The other thing about it, for me, is that blueberries are actually quite small. I get that it needs to be something juicy, but watermelons are juicy, tomatoes are also soft and squishy, and so on.
