Is it acceptable to put the blame on an instructor during an oral exam for a mistake? During class, I asked my instructor about a certain thing that didn't make sense to me. Basically, there was a problem that I couldn't figure out how to solve using the methods we've studied. The instructor told me that the problem couldn't be solved using those methods. As this aligned with my own suspicion, I took her word on it and moved on.

During the oral exam, I was talking about this, and said that the problem was not solvable. The examinator said that it is solvable using those methods, and he explained how.

My response was "ah, I see, that makes sense", and then we kept on discussing.

But now, after the exam, I am thinking whether it would've been acceptable of me to just "put the blame" on the instructor by saying that I got the wrong information from her?

On one hand, that is the truth, and by telling them this, I might avoid being held responsible for a mistake that wasn't entirely my fault. 

On the other hand, and this is how I felt during the exam, it felt to me out of order to bring up the instructor in the exam. She wasn't there to defend herself, so it seemed wrong to talk about her "behind her back", so to speak.
That is an annoying thing that happened to you, and I can see why it would bother you.  University instructors are human, and we sometimes make mistakes, or misunderstand a question from a student, or have blind-spots in our knowledge.  It is certainly annoying to a student if this leads them to error.

However, there is a bigger issue here.  Setting aside the interpersonal issues of whether or not it is advisable to "blame the instructor", I would suggest you take a broader view of your responsibilities as a tertiary-level student.  By university level, it is expected that students are no longer children or adolescents, and they are old and ugly enough to critically assess what they are told by their teachers, and proceed on the basis of rational inquiry and evidence, rather than on the basis of faith-in-authority.  If you are told by an authority figure like a university instructor that X is true, then it is up to you as a student to seek evidence/argument to confirm this claim, and treat the claim with scepticism if there is not a clear and convincing explanation.

In your case, you were told something that was false by an instructor.  Since that thing was false, presumably your instructor could not have backed up the claim with a water-tight argument, had you inquired about the reasoning for the claim.  You say you "took her word for it", but that is not what tertiary-level instruction is about.  By taking her word for it you rejected the standard of reason and instead adopted the heuristic of believing that which is plausible, and is confirmed to you by the assertion of an authority figure.  You paid an appropriate price for that by getting caught in an error in your oral exam, so hopefully, lesson learned.

In this case your instructor probably just made a mistake when she asserted that error to you, or misunderstood your question.  However, in my view, it would even be legitimate as a teaching exercise if she intentionally gave you a wrong answer and then tested to see whether you would parrot it back to her in an oral exam.  The point here is this: you do not go to university to learn to uncritically repeat the assertions of authority figures with PhDs; you go to university to learn to reason rationally, based on evidence and logic.  You failed to do that in this case, so you made a mistake.  That is great, because university is where we go to make these mistakes, get penalised for them, and then learn not to do them again.