Short answer: Yes there is:
[Tenebrium 3] Boosted Weapon = Weapon + Tenebrium Bar (+ Tenebrium Damage (max 1 extra damage type))

Long answer:
There are two aspects of tenebrium and weapons. The first one is tenebrium weapons. They are identified by the word 'tenebrium' in their name, a different meaner look, and a warning that they will cause the rot. These weapons cannot be crafted or improved. Then there are normal weapons enchanted by a tenebrium bar. These weapons use the Tenebrium skill instead of the regular weapons skill (bow, crossbow, single-handed etc.) just like tenebrium weapons.