To clarify: By Tenebrium weapon I meant weapons with the word 'Tenebrium' in its name. If you add Tenebrium to a normal weapon it can be upgraded afterwards. So can weapons with Tenebrium boost as long as they are not named as a Tenebrium weapon.
At least, that is my experience. But there are so many exceptions to the generic crafting rules, so why not to this one too?
Out of curiosity, what is the full name of the Snakebite dagger?