The so-called 'banana posture', in which both the head and tail are held erect at angles to the body, has been suggested as useful in the ID of the species. There is no doubt the species does occasionally adopt this posture as shown in first photo below, but I am doubtful that it is useful in separating this species from Reed and Marsh Warbler, as these two species also occasionally adopt the same posture. I did not note the species to adopt this posture persistently - only sporadically. The species seems to adopt this posture just before hopping onto a branch or taking flight as shown in the second photo below - the same bird as in te first photo but taken two seconds later, and does not adopt it when stretching up to feed as has been suggested - shown in the third photo below.
Every single Blyth's Reed Warbler seen in Goa, of which there were many, were found by their fairly loud 'tak' call. This suggests the species is very vocal, but there were perhaps an unquantified number of birds present which did not call, and which were not seen. The species sometimes interupts it's 'tak' call with a rolling typically type churr, a call often given by Reed Warbler.
The tone of Blyth's Reed Warblers is said to be a supporting feature in identification, but great care is needed. The colour tone of any individual can change instantaneously, and depends on such factors as the intensity of sunlight, camera settings, shadows of leaves and intraspecfic plumage variation. Even after taking all of these factors into account, each observer could potentially interpret and then describe colours differently due to the subtleties involved. The collage of 4 photos below shows the variable colouration of the species.
Other ID Features
Other features I looked for included the extent of supercilium in front of the eye, the primary emarginations and primary projection. These are all useful in separating the species from Reed Warbler, but accurately interpreting them in the field is very difficult and possibly not of much use - either good quality photos or a bird in the hand is probably the only way of recording these features correctly.