I have heard of a purple heroin being attributed to brorphine rather than fentanyl, but whatever it is it is certainly some synthetic opioid, be it fentanyl or some fentanyl analog or some other potent opioid.
Whichever the case, any "heroin" sold on the street level in the US is almost definitely going to contain at least some synthetic opioid, and of those, almost all will have a synthetic opioid as it's main active. What is weird is that some samples contain a small amount of actual diacetylmorphine but so little relative to the amount of fentanyl (or fentanyl analog) present that it doesn't make any sense why they mixed in actual heroin to begin with. They justed wasted the heroin essentially.
Anyway, the purple stuff from what I've heard, is actually brorphine. But whatever it really is you can be sure it is a potent synthetic opioid that can easily kill you in a totally unexpected fashion.