Ok so it's pretty self explanatory why Nashandra's soul has a "dark" appearance, (she is Manus, and Manus' soul was almost identical) but why do the Throne Watcher, the Throne Defender, and Vestaldt's have a similar appearance? Weren't they working against the queen?
The only time she shows herself to you is after you have reclaimed the King's Ring, (killed Vestaldt) and cleared the Throne of Want, (defeated the Throne duo). So if they were corrupted by the dark, would they not be working to help Nashandra?
Artorias (in DkS1) was corrupted by the dark, and his soul has a similar appearance, but even he had the sense to attempt to ward you off, in hopes that you would not fall to the Dark, before he was completely overtaken. Source: Cut Artorias Dialogue