Hey, since Vervaina already answered the majority of the questions - I will just link a segment from Blood Knight guide written by Aerilen re. faith and belief in Light.
Guide: Useful threads: setting, story & roleplay 📖 - #3 by Rogmasha-argent-dawn
Faith in the Light certainly wasn’t dead in Silvermoon or Eversong, if a paladin character clicked on a priest they would dismiss them with “I don’t have the time for a sermon now”. Not to mention the priests were actively working -with- the Light to help Silvermoon rather than using it to help Silvermoon.
“Already the rangers speak of your blessings with awe and respect. Now you can see how the Light serves us, allowing us to help others, but only after we have helped ourselves.” - Ponaris, Quest: Cleansing the Scar.
While there may certainly have been a minority of knight which may have followed the Light still, the majority were disillusioned and disenfranchised like Lady Liadrin.
However, since the renewal of the Sunwell the reverse seems to slowly be coming true; the revised quest texts for example lack a lot of the open hostility towards other Paladin orders who revere the Light, and towards the Light in general. However, due to the Sunwell radiating holy as well as arcane magic now, you don’t need faith to channel it.
Edit/Big Update: A faithless Blood Knight concept is just as valid as one with faith, though one who still clings to the mindset which was apparent in TBC may turn a few heads, not only that but they still suffer the same headaches and emotional instability that was caused from leeching from M’uru:
https://twitter.com/MickyNeilson/status/669361548582739968
Hi, question! Are there side-effects to drawing from the Sunwell like we saw with M’uru in TBC?
Yep! Side effects were described in Blood of the Highborne.