I'm talking about more about imagery, because the visual aesthetic was what was being presented. Soulsborne is more muted, and strikes a fine balance between practical and rule-of-cool. BB leans more toward the latter than DaS. Both 40k and Soulsborne revel in grittiness and grotesque aesthetic, but Soulsborne is more organic. They take inspiration from stuff like wilted vegetation, twisting tree branches, and congealing magma. Stuff that normally doesn't seem creepy until you take a nice, long look.
40k is more obtuse in its designs. Stiff edges, acute angles, and radical protrusions, and lots of tassels and wires. The grittiness and chaos in 40k is mostly conveyed through abundance and exaggeration of details.
Soulsborne's beauty is akin to admiring a mass of thorns climbing up the side of a burnt cathedral. 40k's beauty is admiring an expansive, post-apocalyptic junkyard that happens to be the backyard of a cathedral.
The point is, I don't think they'd mesh well. Though I genuinely wonder how From would do a space-souls, visually speaking.