Thank you gents for sharing thoughts!
I was almost convinced that it’s a Hainan blue. But if the plumages on the breast, belly and under tail can still be seen as grey against the sunlight, Hainan blue’s wings and back should still appear to be blue (rather than black).
Pied bushchat is possible but the one I saw seems lacking the white patch on the wing.
Anyway, I will keep Hainan blue, pied bushchat and also Jerdon’s bushchat in mind next time I check out that place again.