Will Dragon Disciple progress the bloodrager bloodline? I've already searched through the forum and couldn't find anything definitive. I'm playing with a fairly lenient GM so it doesn't necessarily need to be RAW but if the answer is RAI it'll need to have good reasoning behind it. Also, how would eldritch heritage and a robe of arcane heritage work with it?
Just wait a little bit till ACG errata. It will be in a week or two. Then it will become clear. (We here hope so) Currently per RAW it's not allowed. RAI - who knows.
Some people say it's overpowered, but it's not. If you want to take it right now: GM is your only answer.
EDIT: Regarding robe: I believe it does work. Some people disagree. There is no definite answer. Another rules stalemate. :( And this one doesn't look to receive clarification anytime soon. Though in these forum (as it seems to me) more people believe it works.
What I am mostly curious about is if I can be a non-draconic bloodrager and take dragon disciple without switching bloodlines.
You can't.
If the bloodrager takes levels in another class that grants a bloodline, the bloodlines must be the same type, even if that means that the bloodline of one of the classes must change. Subject to GM discretion, the bloodrager can change his former bloodline to make them conform.
DD grants a bloodline via Blood of Dragons so if you take DD you change bloodrager's bloodline to draconic. But nothing prevents you from being crossblooded.
The bloodrager can be any bloodline, the DD doesn't give a bloodline, just the powers of the bloodline like eldrich heritage. So you don't need to be draconic.
If the dragon disciple does not have levels of sorcerer, he instead gains bloodline powers of the draconic bloodline, using his dragon disciple level as his sorcerer level to determine the bonuses gained.
Eldritch Heritage doesn't give you a bloodline, just selected powers from a bloodline. Other than not being able to take powers from a bloodline you already have Eldritch Heritage doesn't interact with Dragon Disciple in any way.
The Robe of Arcane Heritage specifically states that it modifies Sorcerer levels, so RAW it does not effect Bloodrager bloodlines, but might be argued to effect the bloodline granted by Dragon Disciple if you don't already have a bloodline when you qualify (as a Summoner or Bard for example). Robes of Arcane Heritage may also effect powers granted by Eldritch Heritage.
I was asking about eldrith heritage and the robe, not eldritch heritage and DD. Anyways, thanks for for he answers guys.