This is not a well-documented species, but I know because, according to Kyle at www.roachcrossing.com, his population is parthenogenic. In the wild, there very well may be bisexual strains, as found in P. surinamensis in certain areas. I don't know. However, for the purpose of the hobby, it's best to simply say that they are an all-female population, as this is what's been documented and what is available to us.
If the captive population WAS bisexual, the females might mature without wings, as seen in P. femapterus, or have shorter wings which do not cover the entire abdomen, as seen in bisexual P. surinamensis strains (this may also be the case for P. striatus; I'm not sure). Basically, Pynoscelus sp. are documented to be sexually dimorphic when bisexual and so we may assume that our P. nigra are not bisexual as males and mating altogether do not appear to be present.