The Saintly Blessings are specific to St. Celestine, not the detachment. So, Castellans of the Imperium isn't required. Also, the Blessings are not limited to just her detachment but effect any Imperial non-vehicle unit within 12" of her. Therefore, Indomitable Faith could be used for Imperial Lords of War from her detachment or any other detachment within your army.
So, let's say you selected her to be an HQ for an Ultramarines CAD. Then she could grant IWND to Marneus Calgar.
Or maybe St.C is an HQ for Castellans detachment and you've allied in a Grey Knights Nemesis Strike Force. Then good ol' Draigo could gain IWND.
To me, the main issue is points committed. IWND on Draigo is a pleasant bonus for a Draigo/CentStar but that's 685 pts min for Draigo, St.C, and 3 Cents w/grav. That's well over a third of the points of an 1850 list for a single "trick". Which may be worth it if St.C is using her time well (maybe leading some Interceptors in a merry chase of smashing things) but too much to simply make 3 grav cannons more durable.
Some people freaked out about how much durability St. Celestine adds to a Deathstar. But they seemed to have forgotten or not noticed that you'd have to drop 200 pts from that list to put her in. Could still be worth it but some were simply adding in her special rules without sacrificing any of the Deathstar or its supporting units.
Anyways, I hope that helps.