Dante being the sergeant is pretty vague but fairly obvious, if that makes sense
"A few amongst the Chapter Council argue that he is the coalescion of the Primarch's nobler side. the part that kept Sanguinius' darkness In check, and that was lost to the Blood Angels upon his death"
It would seem that Azkaellon wouldn't be immune to psychic blast effects
"The Inquisition in particular worry that the Sanguinor is some form of psychic construct, and that his existence proves the Blood Angels to be just as flawed spiritually as they are physically."
To me that sounds like not a being, but a force made manifest which helps them at their direst need, and when does the black rage come? Thats right, in their direst moment.