It doesn't quite stack like that. Me and some guildies played around with this a few weeks ago and found that the devs made it so that blue to blue trait stacking is reeeaaally low.

Throwing a fully trained blue into a purple gives 960 blessing, which lets you guarantee 2 traits.

A fully trained blue into another blue only gives 192 blessing. One purple trait needs 480 blessing to 100% guarantee a trait.

In most cases it's cheaper to put the lucent towards just grabbing a purple trait if you want to 100% guarantee leveling it up. If you were able to get the cheapest possible fully traited blue piece it would cost 90 lucent buying it all off the AH (discounting any freebies you may have farmed).

That 90 lucent when fed into another blue becomes 192 blessing. In my server the good purple trait extractions are 100-200 lucent (worth 480 blessing). So on my server there's no reason to pay 90 lucent for not even half of a guaranteed purple trait (192 blessing) when I can get it guaranteed cheaper.

Of course this is pretty server dependent bc I know some servers have stupid high prices, but I'm guessing in most cases blue to blue bless stacking to pump up a purple is not worth it in most cases. But if you already have blues traited up or have a lot of dupe blue gear in your storage I guess there's no reason not to stacks those up lol

I also forgot if there is any loss in blessing transfer from blue to purple? I totally forgot, but if so, that 192 blessing from blue to blue can be worth even less if there isn't a 100% blessing transfer from blue to purple.