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"I blame myself for failing the test"

I = Subj
blame = Verb
myself = dir. obj.
for failing the test = Prep phrase + gerund + obj.

Someone told me that the prep phrase is an adverb answering "what"? I was told that adverb prep. phrases answer When? Where? How?
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In my opinion it's adverbial. The verb is "to blame." What do I blame? I blame myself. The question "what" would be answered by identifying the direct object.

How do I blame myself?? Why do I blame myself? (This test isn't perfect). I'd say it's closest to "how." (for failing the test)

I'd be inclined to suggest that "someone" is wrong.

She's probably thinking about the verbal "failing." (failing what?)
Again, the answer to this question gives you the direct object, "the test," which is not adverbial.

It's kinda tricky how the gerund acts like a noun (object of the preposition) and a verb (taking a direct object) at the same time!