My borrowed headcanon from kita-ysabell is that trolls are born in equal proportions - any ideas of caste rarity are purely artificial results of culling programs.

If wigglers are tested in the caverns for fitness, early intelligence, beauty, then it stands to reason that the conditions would be more strict for blues. Thus the great lie of the hemospectrum is that there is little fundamental difference between a rust-blooded urchin and a blue-blooded noble, only that there is less room to fail for the higher castes.