It's not necessarily a nerf, I mean Darius can be healed for more now. It more or less separates the good Darius's from the bad, not by much but it does. It encourages better positioning. Besides, he had it coming; that heal will cause anyone to lose a trade against him, and it isn't that even huge of a nerf for laning phase, % based abilities mean less earlier, and more later, and he is getting an increase on total % HP healed.
Did some math. This is all suggesting Darius heals from his Q at exactly half of his base HP.
At level 5. Half of Darius's health is 445.5 HP.
Pre-nerf heal: 66.8 HP
Post-nerf heal: 53.5 HP
Difference: 13.3 HP
At level 18. Half of Darius's health is 1,141 HP.
Pre-nerf heal: 342.3 HP
Post-nerf heal: 410.8 HP
Difference: 68.5 HP
Pretty sure 10 HP less healed early game compared to a near bonus 70 HP healed is more of a buff than a nerf.