[Mojo the Avenger]

That's possible, and if you decide you want to cast rejuvenate instead of build boots of youth it's the faster option.

Horde from Hell is at Blood 5, if you go to Construction 6 then you're at Blood 0. It takes a lot longer to get to from Blood 0 to Blood 5 than from Thaum 2 to Thaum 4.

Not to mention 44 blood slaves is a lot, and 15 nature gems isn't so bad.

Depends on your research.