Just as a warning, this post will contain some spoilers from the final Fallen Kingdom trailer.
The reason they probably want Blue specifically is because Blue is birthed through the IBRIS Program, which genetically domesticated the Raptors by engineering severe aggression and unpredictability out of their genome. Utilizing Blues DNA would make the iRaptor more docile and thus more willing to take orders in military application. They have a whole manner of species DNA profiles on hand. So I imagine that it's because Blues DNA is special. They're also discussing animal trade exploitation in this one, since they're selling the iRaptor to the highest bidder, imagine owning a slightly less aggressive raptor. It'd be like owning a panther.
Another thing to consider is the grow cycle. We're theorizing that Blues DNA is mixed with that of the Indominus to create the iRaptor. But it's possible they're using advanced growth hormones to increase the growth rate of the iRaptor. If this is true, then unlike Blue bonding with Owen and gaining a positive relationship with him, the iRaptor has no relationship formed with humans and thus has no idea what they are and is instantly afraid (and in turn, aggressive) of them. This would continue another side of the Frankenstein story where the monster was fearful of humanity, yet also curious of it and slowly learned civility through observation. If this is the case, then the scene where the iRaptor is investigating Masie in her bed directly mirrors the scene in Frankenstein when the monster curiously investigates Dr. Frankenstein while he's sleeping in his bed.
I think I'm going to like Fallen Kingdom.
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SOMETHING HAS SURVIVED