One of two questions regarding the Sorcerer
I have always understood that the Crossblooded Sorcerer Archetype cannot replace any of their bloodline powers with a bloodline mutation
Can Wildblooded Sorcerers?
The Primal Bloodline replaces the Elemental bloodlines 9th level bloodline power. If I take the Primal Wildbloodline must I take 'Elementalist Summoning' or may I still replace it with a bloodline mutation?
That depends upon whether a Bloodline Mutation is actually considered an archetype in all but name (analogous to what Thassilonian Specialist Wizard apparently is), or whether it is just another substitution (analogous to Wizard Arcane Discoveries, which are considered Wizard-only feats, which thus can be taken even by Wizards that alter or replace Bonus Feats). If the former, then neither a Crossblooded Sorcerer nor a Wildblooded Sorcerer could take them, although one could make a decent argument that a Wildbooded Sorcerer could take one of these that didn't replace the same Bloodline Power that the Wildblooded Bloodline replaced. If the latter, then either one could take them. I would lean in favor of the latter, but then again I also think that making Wildblooded Bloodlines be treated as arguments was an unnecessarily restrictive move that also plays havoc with the organization of Sorcerer Bloodlines.
"a bloodrager or sorcerer cannot swap a bloodline power that she has altered or replaced with an archetype for a bloodline mutation." MTT pg. 10
Wildblooded is an archetype, and thus any replacement to any bloodline power the wildbloodline makes prevents swapping it with a mutation. Nothing prevents picking up a bloodline mutation in place of a bloodline bonus feat, though.
@UAE: Mutations are alternate class features akin to AWT or Arcane Discoveries, and the interaction with archertypes is solely dictated by what I quoted above.
^Somehow I missed that part (which implies the answer is No since Wildblooded is considered an archetype).