I think it will be Spring Hill. Admittedly I'm not some industry insider, but from everything I've heard and read, the Blazer is going to be RWD like the Lyriq and sharing a very similar design. The Equinox, I suspect, will be FWD and AWD, and will be built alongside the Buick crossovers, wherever that may be. Maybe Mexico?
We will see... Half of Spring Hill is set up for Lyriq, and the Honda/ Acura similar model, the other half of Spring Hill is still building Ice vehicles, and the Blazer is a much smaller (less premium) vehicle than Lyriq. It's possible it will be built in Spring Hill, but I doubt it unless the second half of Spring Hill dumps its ICE programs. Spring Hill does make sense though as they will be able to take advantage of the battery plant location, and the potential USA built tax credits upcoming.
Now, will GM move the last ICE programs from Spring Hill to Lansing Delta or other plants? Lansing Delta has a lot of construction work ongoing currently, and so that is a possibility?
Now on the Mexico thought, we start with GM's billion dollar investment they have previously announced will start building EV's in Spring 2023 (timing lines up), Blazer and Equinox are currently built in Mexico (product lines up) also on an dedicated EV assembly line, does not matter FRONT, RWD, or AWD as all options are installed into the skateboard as a front and rear substructure. for example Hummer (BT1) could be built as a FWD, RWD or AWD on the same line...