This was already extensively discussed here :
www.elitefourum.com/t/can-blister-packs-be-weighed/23456/23
The TLTR is that, unless a blister pack is extraordinarily (i.e. unicorn) light or heavy, it cannot be said with complete certainty that the pack it contains is (respectively) light or heavy. The heavier the better, but we cannot quantify at all how better. Is 34g a lot better than 33g or a little better? We can’t tell.
Blister packs taken from a same case and stored the same way can be weighted, but it is presumed that this practice wasn’t commonplace (at least for Base 1st) and, therefor, blister packs in the wild should be close to 1/3 chance at heavy.