Of course, we know that translucent ventral scales/areas are not present in all adult Bloods (none of my adult Bloods/Blood based Corns have them). Thus, it is a fail, as an indicator, for adults.
However, there does appear to be a high liklihood, of translucent ventral scales/areas being present, in Blood Hatchlings.
Although, as you seem to have found, it does not seem to be present (or, at least, not obviously present) in every single Blood hatchling.
I took a look at my 2011 Bloods, & Blood Based, young'uns. While a high percentage had some translucent ventral scales/areas, it was not obviously present with 100% of them (and there is ~no doubt~ they are all Homozygous Blood).
Perhaps it would be obvious in 100% of freshly hatched Bloods? Especially since it seems more prevalent the younger that a young'un is.
I don't really know if it would be.
Will be something I need to remember to check, in my freshly hatched Bloods, next year.
'Course, I am going to be taking a look at all non-Bloods too.