gary

I would say it's not a Hainan Blue flycatcher, as you can see the throat is clearly pale contrast with the orange breast.
Most likely, it belongs to the complex of Blue-throated Flycatcher藍喉仙鶲/ Chinese Blue Flycatcher中華仙鶲, and I bet it's a Chinese Blue Flycatcher Cyornis glaucicomans.  The status for this flycatcher complex is very uncertain in Hong Kong.
By the way, where is the photo taken? It's very useful to assess the status of this bird.

Gary