if Mettaton really wanted to stop us on the geno route he shoulda stayed in box form (/hj)
Well, if Gaster didn't make the blueprints then who the fuck did?
made the MACHINE, not the blueprints.
alphys made the machine, and Gaster made the blueprints.
she made it after Gaster fell into the core.
making the blueprints means well. making the blueprints.
it does not Inherently mean you've actually gotten around to making the thing in real necessarily. They're just instructions. (though prototypes of inventions are certainly a thing, so when I say 'neccessarily' i mean it could go either way.)
If Alphys phrases things as 'using the blueprints' when describing things, that could either mean she built the DT Extractor based on the blueprints (which in my opinion is the most likely option), or I suppose in the case where you believe she didn't build it, utilized the reference materials on the blueprints for the operation of it.
I thought everyone knew the blueprints thing...sadge
Entry Number 17 suggests his creation was a Dark World, which could've brought the DT Extractor to life as a Darkner.
I could see an argument for a prototype having existed and then funny darkness stuff happening!
The fact remains Alphys would have then built A DT Extractor from the blueprints. :3
As an aside : people are really underselling the ramifications of Gaster being interested in such a way in funny number 17!
[gaster voice] I don't know... that's INTERESTING.
A DT Extractor that extracts DeTermination instead of DelTarune.