I don't play DRG, so this doesn't mean much coming from me, but...

In regards to Blood of the Dragon and associated skills, it's implicitly the WoL drawing on his/her bond with Midgardsormr, not Nidhogg. BotD gives you energy tendrils in Midgardsormr's blue/white energy signature, which appears both when using Dragonsong Dive and when you fight the phantom Midgardsormr at Keeper of the Lake (though how a DRG can use Dragonsong Dive before then is a big question - then again, the chances of LB3ing anything at that point in the game are pretty null right now).

Conversely, Nidhogg's energy signature is black/purple. It shows up every time Estinien pulls out the Eye, surrounds Estinien-Nidhogg when he's using his powers, and is prominently displayed when he uses Final Chorus (using both the energy tendrils from BotD and then some). Hraesvelgr's energy signature seems to be a golden color, as shown when his Eye transfers its power to you before the Final Steps of Faith.

As for Soul Crystals, implicitly anyone who is capable of resonating with a given Crystal can learn the skills within it. NINs pass soul crystals down in their hidden village for this express purpose. What makes us special is that, implicitly, the Echo and/or Blessing of Light lets us resonate with any and all Soul Crystals. So while others have to train for a lifetime to develop their skills, we just hack into Soul Crystals to become proficient in the arts we can go on a deicide binge in an afternoon.