Darius can heal when Taric uses his ult and is invunerable

Pretty sure it is, since it heals on-hit.

It is working as intended I believe.

http://leagueoflegends.wikia.com/wiki/Darius

Taric's ult would disable LIFE STEAL since you wouldn't do damage, there would be no life steal % sustained. any number multiplied by 0 is 0. 0 damage x 20% life steal is still 0.

DARIUS Q though is NOT life steal - it is "restores % of my health if Q hits." So even if it does 0 damage, it heals % of his max health. The ability name is Decimate.

It is intended. By virtue of merely hitting you, darius will heal, regardless if it does 1000 damage or 0 damage.