CC1, The Oxford English Dictionary generally tries to establish the earliest version of phrases in English. As regards fill in/out with forms etc, here are the two appropriate quotes...
"Mr Lanman forwarded to him a blank to be filled out with facts and dates." (1880)
"He had left the date blank for the plaintiff to fill jn." (1893)
It seems, therefore, that both are from the same era and equally valid, so nothing much to detest, really.
On the other hand, I totally agree with you that "could of" is a barbarism that ought to be fought against.