Perhaps it should be said that singing cannot really be taught but it is down to each singer to let his/her own voice evolve using their own voice, and what they are comfortable with
The singing style may start off of being a 'mimic' of the source singer and there are certainly singers who sing material where it is pretty clear where they got the song from as the source singers intonation is clearly there
The singer should let his/her own voice come through and the style should evolve, to my mind influenced by listening and absorbing but using all the techniques which the singer can or feels apt
This also brings into being the singing of material which is really derived from the singer's own area
A bit McColl school I suppose but clearly English singers should not sing Scottish songs or Irish songs which are basically out of area in a pseudo accent (dont do as I do but as I say!) ideally