To me, this looks more like a Richard's Pipit, based on the strucutre/jizz. Overall, the bird seems very slim and long-tailed, almost like a Yellow Wagtail. The bill length is difficult to judge on these photos because the bird is facing slightly away, but seems to me to be long enough for Richard's. The hindclaw is also difficult to see, but seems to be longish and not as strongly arched as I remember from Blyth's.
In terms of plumage I think the bird is fairly plain above and below, not as strongly streaked (especially on the mantle) as I would expect from Blyth's. The underparts show strongly buffy flanks and whiter belly - more of a feature of Richard's I think. The pattern of the median coverts (which is supposedly diagnostic in adults) cannot be seen clearly on these photos, but it seems that the bird is a first-winter without any adult MC, in which case this feature may not be present.
Calls are difficult to judge (and, of course, very difficult to write). Both species have a variety of calls. In addition to the classic 'schreep' call of Richard's, there is also a shorter 'chup' call. It is interesting though that the bird responded to recordings of Blyth's (although you don't mention if you also tested it against a recording of Richard's!)
In terms of behaviour, the Kam Tin Blyth's Pipit was distinctly different from Richard's by dropping rapidly into cover after flying - not hovering over the ground and landing on open areas, as you seem to suggest this bird has been doing.
Having said all that, I have relatively little experience of Blyth's and it must be overlooked in HK. I'm happy to be proved wrong if this does turn out to be a Blyth's Pipit!
