It's a unique part of the series and the nature of the spectral blobs is really only implied. I feel like they act as stand ins for Apostles in this shadow of an Eclipse that Skull Knight alludes to.
The spectral blobs' devouring of the refugees indeed corresponds to the apostles and their devouring of the Falcons — that's a key part of the allegory SK was talking about (as was Guts/Casca/Femto/SK/Zodd's presence).
I imagine it's also unique to St. Albion's mass graves since we never see the proximity of our two branded characters triggering anything like it again.
Like Aaz mentioned on the show, we've seen another unique spectral entity before — the tree outside Jill's village. Humans were sacrificed, trapped there. Over the years, their souls intermingled and associated with the tree, and when Guts awoke it, they manifested from the tree. The same happened in and around Albion. Tortured souls commingled and fermented there for dozens, perhaps hundreds of years. And when they were awoken, they were no longer the disparate, floating specters that haunt Guts — they were souls manifested out of the blood and decay beneath Albion.
I think you could also argue that we saw something similar to this spectral amalgamation in Volume 10, when Griffith was granted a vision of the God Hand.
