They must have changed this in the previous updates. It has been 2 weeks since I've had time to sit down with the game. On the latest build I just got up to getting Tenebrium and the Sword of Planets. I haven't tried to add any Tenebrium to the weapon yet or anything else.
In an Earlier build, adding Tenebrium prevented you from further weapon upgrades, including just sharpening them. I experienced this myself.
