That was changed in one of the last couple of patches for D:OS. The Tenebrium ability now allows you to use weapons with that damage type (and handle the ore/bars) but does not change the main weapon ability or add any extra damage.
I don't know if the EE will make any additional changes (in D:OS, level 3 is needed to add a tenebrium bar to a weapon, but anything above serves no purpose).
