I'm sure he had something in mind, but he still says it's him. He's not trying to pass the blame.
I dare to say its deeper than "who's to blame" , Paul genuinely didn't want to do some things , things he hated to do but did them anyway , so , whilst it was him that did these things , he recognised an intruder , he says he didn't want to do what he ended up doing , and visa versa , he wanted to do much he couldn't do , this frustration made him recognise his will was being attacked by sin , now if its sin not I , then in one important respect Paul is at heart blaming sin , he acknowledged his body was troublesome , "this body of death" and he strongly implies he wants what is best but cannot always get it , it seems sin , death and Satan are seen as enemies not the true desire of any regenerate .
"That which is born of God does not sin" ie , he has a new nature and a Christian sins only by weakness of the flesh not by the new mind , heart or will ...
1 John
[6] Whosoever abideth in him sinneth not: whosoever sinneth hath not seen him, neither known him.
[7] Little children, let no man deceive you: he that doeth righteousness is righteous, even as he is righteous.
[8] He that committeth sin is of the devil; for the devil sinneth from the beginning. For this purpose the Son of God was manifested, that he might destroy the works of the devil.
[9] Whosoever is born of God doth not commit sin; for his seed remaineth in him: and he cannot sin, because he is born of God.
So who's to blame ?
Sin.
Romans 7
[20] Now if I do that I would not, it is no more I that do it, but sin that dwelleth in me