Afraid not. Phormiums like moisture retentive soil and plenty of sun, so if they're short of either of those, that might cause a fading of the colour, but frankly, many of the purple/black varieties do not keep their original dark coloration, often reverting to dull green or purplish green. One black variety which does seem to more or less retain its colour is Platt's Black, in my experience, but I don't think its quite as hardy as the one you have, although that probably doesn't matter where you live.