Soren - I am sorry that your question was ignored here. It may be simply because no one had what they felt was a definitive answer to your question. I have never seen what I would call a documented answer to the black neck seals on some German 9 mm cartridges. Regarding “normality” I have long since given up the term “normal” in describing German WWII ammunition - the “norm” seems to be that there is no norm.
Regarding the all black primer, somewhere in my notes I think I have information that indicates that with the earliest use of the steel cup primers as a standard option, some factories covered the whole primer cup with the colored primer sealant. This was done to prevent rusting of the external service of the cup. The information I have was that it was found out this practive caused a transfer of the sealant to the face of the breech of the weapons in which it was fired, sometimes gumming up the firing pin hole, so a directive was issued stopping the practive. The earlier of the two “dnh” 9 mm rounds I have with the black neck seal has the all black primer, while the later of the two has only the normal ring of black sealant around the outer edge of the primer and the inner edge of the primer pocket.
Regarding the black neck seal, the only rounds with sintered iton bullets that I have with this are from RWS (dnf) and DWM (faa). They have the ring of black sealant at the primer as well. I don’t collect lot numbers and dates, only visual differences, so I don’t know how common (that is, how many lots) from these two makers have this.
I think your guess on the British forum may have been accurate. We all know of the commercial DWM, Geco and RWS pistol ammunition with red seals at the primer and neck, which were to keep out moisture and, I suppose, other contaminents, so why not the black ones on the SE-bulleted rounds for the military, as well? For me, it remains a guess, since again, I have seen no documentation of this use of the neck seal on SE 9 mm 08 rounds, but it is certainly a logical guess.
Again, sorry you felt ignored. I was going to let someone more knowledgeable than I answer your question, as there are so many on this Forum that know this stuff better than I do. That, coupled with the fact that I have no documentation on the subject, but felt perhaps others did, kept me “off” the question until I read your entry on the site provided by EOD.