To be honest, I've never understood why elans are considered aberrations in the first place. Basically, they are a symbiotic organism in which their spirit is actually a psychic entity in its own right that is no longer able to survive outside of the body. As I recall, the background of the character must include having gone through a ritual that effectively 'awoke' this 'true self' - in the process erasing all previous personality traits / skills / classes / etc as the second personality gained ascendence, dominance, and completely fused-with / consumed the former personality.
In effect, the spirit / mind of the character is replaced with this 'true self', granting the character psychic abilities while permanently removing all prior traits and levels (if any existed, from the text it seems most are only first or second level when this occurs).
So why is this an aberration? We have a body with a soul / mind within it. At best, only the qualities of the soul are 'aberrant' - and that only in the fact that it is more of a psychic ghost that has taken permanent possession of the body and undergone a spiritual variant of the 'fusion' power.
Their body is not aberrant, so I would base my decision on (1) what is gained from the feat 'aberrant blood', and (2) what further feats the PC wishes to take. If they fit the pattern of the elan, then I would likely allow it (while not allowing the gain of th feat not taken). If the feats the player intends to take, however, have little in common with the nature of the elan, then I would not allow it (without first taking he 'aberrant blood' feat, that is).