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I have never seen a Dalmannia, but of the three species known for Berlin/Brandenburg (D. dorsalis, D. marginata, D. punctata) only D. marginata is thus black. In the forum insecte.org that is a very informative one, there is a quite similar thread: https://www.insec...hp?t=76523. There is also a key by Stuke (2007, https://www.contr.../1690/1689) for the 6 palaearctic species that leads to D. marginata. So I think the species is ok!
Regards, Sundew