I love ADB to bits. I think Cadian Blood is the weakest of anything I read by him. Nothing too exciting. The plot or situation isn't all that interesting. The characters are pretty plain and not very fleshed out.
I wouldn't be surprised if they just stamped ADB's name on someone else's work to get it to sell. That's why I bought it :p.
I take it you're not aware that Cadian Blood was actually the first full novel written by ADB for Black Library, then. It was this, Soul Hunter and Helsreach (in that order) before The First Heretic.
If I may speculate, it was probably based solely on Cadian Blood's positive reception that Aaron was invited to join the Heresy team (the other two novels weren't even released when The First Heretic was announced, if I'm not mistaken)
At any rate, it's fairly certain that there is no conspiracy, and Cadian Blood wasn't actually written by someone else, in order to capitalize on his success
EDIT: I agree that every other book by ADB is better than Cadian Blood, but you can't blame him for that. First of all, he was probably constrained by editorial demands on his first book. Secondly, he was just starting, back then. It was probably a difficult stage. Trying to deliver results without taking too many risks. Gauging interest for deeper themes, as opposed to regular bolter porne... You know, the sort of things that plague every new beginning... Since then, he's been very in touch with the fan base, very quick to adapt to feedback, and you have to agree that every improvement is visible.