There is absolutely no way to distinguish a Blue Gene Dempsey from a standard Dempsey...
As mentioned, if you buy two Blue Dempseys from the same source, common sense says they will be siblings and will not offer the diverse genetics that Blue Dempseys need...
Blue Dempseys have been bottlenecked in the hobby for quite some time so if a healthy breeding system is desired then you need to do the research and acquire fish that are as distantly related as possible.
If I sold a sibling pair to a guy in New York and another sibling pair made from the same brood to a guy in California... and you buy one from each... you simply doubled your shipping costs with no benefit... Do your homework
There are a handful of people who are putting respectable efforts into breeding better quality Blue Dempseys... my suggestion is to join them or leave the work in their hands... producing more low grade Blue Dempseys will only further spoil the reputation of this beautiful fish...
Note: I am not suggesting anyone here is attempting a low grade project... I'm simply sharing my opinions of what I've seen over several years... and am not directing comments "at" anyone here...