"109. For census purposes, the term "black" (B) includes all persons who are evidently full-blooded negroes, while the term "mulatto" (Mu) includes all other persons having some proportion or perceptible trace of negro blood."

That was the guideline...
It does use the word *evidently*...So someone who appeared to be full Black didn't have to be proven to be full Black to be classed as Black.

In my post you quoted from, I was referring to census takers being accused of writing down that someone was Black or Mulatto when they were actually pure white, or white and Indian only...
However...you said it was common knowledge that the enumerator's half sister in law had a European father...Have you found official records of that? And if so, was he a white european? I'm not asking you to prove it to ME..I'm just wondering how it was common knowledge then or now..