Does the Bloodline Arcana from the Psychic Bloodline deny meeting the prerequisites for the Arcane side of the prerequisites?
Particularly with a Crossblooded Sorcerer?
I'm not asking in the rules section because it's probably a hard NO.
But from an RP perspective, does it fundamentally change anything that matters?
Well, from an RP prospective lets look at the 3 different sources of magic.
Arcane magic takes advantage of magical forces in the world. It draws energies both from the world the caster lives in, and from other dimensions by taking advantage of arcane rituals and cracks in the fundamental laws of reality.
Divine magic entreats powerful outsiders and fundamental forces to act on behalf of the caster. Most are a patron and supplicant relationship. The patron isn't always a god, but most are. Sometimes spirits, outsiders, and even ideals can be the patron that reacts to the supplicant's desires.
Psychic magic relies on the caster to reshape reality. While the manifestations are similar to other forms of magic, other forms of magic rely on external forces to empower their magical effects. Psychic magic relies on the self and the self alone. This makes psychic magic fundamentally different from the other two forms of magic, and alchemy.
Alchemy isn't magic, but rather the user understanding the relationship between nature and magic. Manipulating natural forces the alchemist is able to cause magical effects to be created. This is similar to arcane magic but the user doesn't attempt to internalize the forces and creates an alchemical medium that serves to do this in the alchemist's place.
So, does that clear anything up?
Do you see prestige classes as something which build on what others have done and learned? I do, and if so then there may not be such a foundation for a psychic mystic theurge. It depends on just how widespread in your world psychic spellcasters are. If they're rare and/or foreign then there probably isn't the same sort of knowledge of how to integrate psychic magic with other magic out there.
Mind you, if someone really wanted to do it I'd find a way to work with them as a GM.
Well, I was thinking that Psychic powers come from within, much like the powers of your own blood...?
It's literally a Bloodline.
Your Psychic abilities run in your blood.
Some breeds of dogs are smarter than others, inherently, it's in their blood.
I don't see this in any way changing what Mystic Theurge is about or what it has to offer. For RP purposes, I see no reason to not allow it, since you obviously can enter the Prestige Class as a Sorcerer relying on your blood-magics.
But, exactly for this reason, I thought that mixing the Destined Bloodline and Psychic Bloodline, I would be a driving force of personal destiny...
Combined with the Luck Domain from the Divine side...
I agree with Voodists conceptuality of both psychic and blood magic. Which is why I don't think it fits a Mystic Theurge.
Mystic theurges place no boundaries on their magical abilities and find no irreconcilable paradox in devotion to the arcane as well as the divine. They seek magic in all of its forms, finding no reason or logic in denying themselves instruction by limiting their knowledge to one stifling paradigm, though many are simply hungry for limitless power. No matter what their motivations, mystic theurges believe that perception is reality, and through the divine forces and astral energies of the multiverse, that perception can be used to manipulate and control not only the nature of this reality, but destiny itself.
Mystic theurges are drawn from multiclass spellcasters who can already cast both arcane and divine spells, and the powers that they gain increase their mastery over both.
This all seems to indicate they study how to control forces and energy from beyond, not horn their own inborn talents. Purely from an RP perspective I'd disallow oracles and sorcerers from entering MT because they don't learn their abilities through study.
RP is going to be more restrictive than RAW.
Form a rules perspective, it is indeed a hard "no", confirmed by this FAQ.
For RP, I'd say it's absolutely fine, because Mystic Theurges "place no boundaries on their magical abilities", and "seek magic in all of its forms, finding no reason or logic in denying themselves instruction by limiting their knowledge to one stifling paradigm". The description only mentions arcane and divine spellcasting, because that's all that existed when it was printed, but I'd say the "magic in all of its forms" parts would actually be broken when we exclude psychic casting, and I feel that's the centerpoint of the class concept. Flavorwise, that is.
@Meirril: I think you're a bit too focussed on the "knowledge" part. Any magic user (Sorcerers included) requires training, that is what class levels represent. It's probably more learning-by-doing, and trial-and-error, but a spontaneous caster still needs to accrue understanding of how their spellcasting works. That includes, of course, how to control the magical abilities they're born with.