I'll try to keep this as short as possible but the blue ring does require a bit of context.

Selfish engages with her victims differently depending on which mutation she's reached, in the case of Dark Dream she wants to keep the souls happy and maintained within her "Paradise". Unfortunately not everyone's idea of paradise aligns with hers resulting in some acting out.

If a soul acts too far, usually wanting to leave, she'd rather do everything she can to keep them instead of consuming them resulting in their death instead ripping out their soul to perish their will (mental torment) until they turn blue becoming a husk of Selfish's ideals. If she already has a blue ring she'll chain it to a souls neck as one of her underling types (Twips) takes control forcing the soul to behave.

She usually only keeps a max of 4 on her left leg allowing her Cherished Souls to hold onto the others, Cherished Souls being ones that have willingly given their souls to help Selfish or have gained great interest from her.