I wanted to make the darts skill no longer hidden, so I removed <Hidden>True</Hidden> in Skills.xml, but that didn't work, the darts skill is still hidden! What's going on here?

Did you delete it entirely or change it to <Hidden>False</Hidden>? (I would have thought deleting that line would have caused it to default to non-hidden, so if it didn't I'm curious how the default settings actually work)

I know this response is late, but since this is the only place on the 'net that discusses this issue...

I'm currently working on a mod to add icons (and maybe descriptions, depending on how confident I am in my coding skills) to previously hidden skills; while working on it, I discovered that Darts (alongside Ballet, Bubbles, and Hopscotch) are actually hardcoded in the game to stay hidden unlike with the other hidden skills. So you will actually need to edit the Sims3GamplaySystems.dll to actually unhide it.

I agree with you, you are right.