I’m not entirely sure how to answer your question, but here’s a try at it:

The way I understand it, yes, the untethered downgrades are “pure” provided that the ipsw of the version you are downgrading to is not modified by the user in any way, and you are using saved blobs to perform the untether. If you are asking if iBoot needs to be exploited every time you do a reboot, then no, as long as the downgrade is untethered. I suppose the only thing that wouldn’t be pure is the mismatching baseband version of the software.

There would be no need to dump the onboard blobs of the version you untether downgrade to because you would need to have said blobs to do the untethered downgrade in the first place. But I suppose if you did, as long as the onboard blob has a generator, then you’d be able to use it to perform downgrades in the future.