>>528923797
There is no lore reason for it. You will probably get a reply saying it's meant to be Frostwolf armor, but that is complete fanfiction that is never stated anywhere, anybody that tells you it is Frostwolf armor has no source for that being the case and is making it up. The running theory is that this is a case of the telephone game where the reintroduction of the Doomplate and his braids in BfA had an artist mess up and draw his armor to be the same shade of blue as his braids, and ever since then more depictions of Thrall take that depiction as gospel and keeping making the Doomplate bluer and bluer. This culminated in the Heartlands art where the Doomplate is literally the same shade of blue as Jaina's robes.