Just wanted to point out that in french we refer to a rookie as a "bleu" (so, blue) and, as far as I know, we don't have the same history as Japan for colors.
In Japan it's probably like in English, a reference to fruits that are not ripe yet (=> ao ringo).
Basically bodies with lots of melanin in them; it's a cluster of melanin at the lower back, sometimes mistaken for a bruise by carers unfamiliar with the phenomenon.
[0] - https://en.wikipedia.org/wiki/Mongolian_spot
EDIT: Oh, it's mentioned in the wiki article:
> The Mongolian spot is referred to in the Japanese idiom shiri ga aoi (尻が青い), meaning "to have a blue butt", which is a reference to immaturity or inexperience.
Dictionaries I've seen say that's the case (or some suggest that the "ao" for youth was borrowed in from Chinese). But to clarify, "ao ringo" doesn't mean an unripe apple - the ao just means green there.
As for "aonisai", the etymology there is murky but surprisingly it apparently has nothing to do with two year-old babies. The main theory seems to be that it's a corruption of a older phrase with a similar pronunciation but unrelated meaning.