The perfect case, should not only be blind but even instantaneous. I agree that this last condition, it's generally hard to get... I was able to get instant switching, only in two cases: a CD player connected to same amplifier, by its analog outputs and with digital trough a dac connected on same amp. Cd was playing seamless and inputs switching made possible to compare without any delay. The second case were two mini's connected to two available toslink of an amp and one settled for Spotify, the other for Tidal 44.1KHz. In both cases differences were earable, much more in the case of CD player but this it's just another personal opinion...