These strains that are sold as 'black' or 'purple' are seemingly (at least when regarding forum posts) rare phenotypes of these strains. Now iam not saying that if your plant doesnt come out dark, its not a Black strain, but fact of the matter is the pheno is rarer than just the green colors. You google BlackJack, BlackBerry, BlueBerry, and other 'purple/black' strains, and you find plenty of sellers showing pics of them showing these color types. Browse those same names under this forum, and you find a lot more 'its not expressing the purple pheno' posts.
Its all genetics, and with genetics, it comes down to chance. Some strains express these traits far more often than others, depending on the bank, the strain, the way the seed was produced, environmental factors, and whatnot. There ARE some strains that people say they have seen the expression more often than others, PPP iam TOLD shows it more often than not. I have yet to see a post on this forum about BlackJack that truly came out anything near purple/black (even though its parents are supposed to be BlackDomina/JackHerer or BD/Jock Horror, depending on who you talk to).
As far as i know, i heard that Black Domina (like pure Romulan) is clone only, so either sensi is wrong, or my info is wrong.