Take inspiration from...
(Some great places to take inspiration from when trying to figure out what kind of shapeshifter blu can be!)
Author's Notes
Images may contain some level of body horror, to the extent of multiple limbs, extra mouths, additional body parts, and possibly unsettling abilities.
https://66.media.tumblr.com/45a5083f80205aa6ae025c8ae9a47fdf/tumblr_mwad3yXWrM1r9hc42o1_r2_400.gifv
https://media1.tenor.com/images/6ad47322ae8b7bfa2e158e3ee077871a/tenor.gif?itemid=12661963
http://rebloggy.com/post/blood-gore-body-horror-too-bold-the-beast-too-bold/45649316077
https://www.deviantart.com/monochrome-telegram/art/Body-horror-shenanigans-771135853
https://www.deviantart.com/odvunir/art/painting-comm-wip-body-horror-creepy-eyes-warn-548575880
https://qph.fs.quoracdn.net/main-qimg-a37e9583878ee3c905f834d88cbcbc8e.webp
If you google "deviantart body horror" there are more good examples