The male Blue Rock Thrush is unmistakable, with all blue-grey plumage apart from its darker wings.
Females and immatures are much less striking, with dark brown upperparts, and paler brown scaly underparts.
There are five recognised subspecies of the Blue Rock Thrush.
This is Monticola solitarius philippensis that occurs in Japan.
The male is different from the other subspecies by having a rufous-chestnut plumage from the mid-breast down to the undertail.
There is a proposal to split the Japanses subspecies into its own species: the Eastern (or Asian) Blue Rock Thrush (Monticola philippensis).

1st, 2nd & 3rd photo: Nigishima, Mie Prefecture, Japan, 28 September 2018
4th & 5th photo: Yunomine, Wakayama Prefecture, Japan, 30 September 2018