Blue weapon still ok to upgrade awakened siren/true siren... heck you can even use Purples (make sure its Pirate/Warrior 7fl mushin/Blackwyrm weapons) those purples give the most Maximum exp (though its rng... if your lucky you get 1 lvl worth of upgrade with just 1purple + 2 soulstone)
awakened pirate/true pirate (its better to transmute at this stage) and if your lucky you might get 4pcs Elder Dragon Element (they are tradeable and you can sell em for 4-6g per piece)... though as far as i know... Elder dragon only has chance to Pop up if you transmaute the Bundle one (the one that needs 15ss + 1.5g)
Do the transmute elements give more exp than the normal element of luck that you can get from the daily dash atm? Because those give 1/4 to half a lvl at awakened siren stage, if those are the same it's not cost much Blade And Soul Gold. Transmuting element package with current soulstone prices costs more than 6 gold which in the worst case would be 3/4 of a lvl and in the best case 3 lvls. Feeding blues costs around 1-2 gold per lvl thus is the better choice if those elements are the same..
You can make 19 soulstones (21 with the PK quest) from doing faction dailies (Woods, Beach and ToE daily) not to mention SS from PvP. That's enough to transmute an element package (which do provide more experience than Elements of Luck as Ive noticed, especially if you receive the Elder Element). How much so, I don't remember. In the long run, it's far more expensive to feed blues into the weapon.
If you were going to transmute the soulstones in the first place to make an element/package, why would you even consider their value? By buying soulstones and then transmuting, you are losing money. You can easily get the required amount of soulstones to transmute elements through dailies and PvP. Meese failed to mention that Elder Elements/Jewels are tradeable, and go for a nice bit of gold if you sell one on the market with Blade And Soul Power leveling. You save and potentially make more of a profit getting the soulstones yourself and transmuting packages than you would ever get feeding blues for small exp gains.
Element packages give you anywhere from 2-4, with a potential one being sellable on the market for an average of 3g on the NA servers. It only costs 1 silver per element as opposed to a handful of silver with one blue. You are not saving money by feeding your HM weapon random blues you find. You should be salvaging those for element powders to make your overall progression faster. I don't really know how I can word this any simpler: feeding your weapon other blues stops being cost effective when you hit Awakened Profane.