People often confuse the meaning of the word "evidence". They say an observation is "evidence for" a theory or "evidence against" a theory, as if theories are tested in isolation.
Proper scientific analysis requires that there be multiple theories, and that experiments are performed to distinguish between theories. In this case, any observation is either "evidence that theory A is better than theory B", "evidence that theory B is better than theory A", or is neutral on the two theories.
Thus, one does not look at a black crow or a yellow pencil and claim either is "evidence for the theory that all crows are black." For an observation to be evidence, there must be multiple theories to compare the observation against.
Thus, when comparing "all crows are black" to "all crows are not black" or even "87% of crows are black", then observing the color of a crow will definitely be evidence favoring one or more of those theories over the others.
"Compelling evidence" is a set of observations that exclude one or more theories as very unlikely. Thus, looking at a single crow and seeing that it is black is compelling evidence to exclude the "all crows are not black" theory, but it is not compelling to distinguish between the other two. It would take observation of dozens of black crows to provide compelling evidence against the "87% of crows are black" theory.
Whether a set of observations is "compelling evidence" is subjective in the sense that different circumstances call for different levels of exclusion. For this reason, scientists often clarify their evidence with phrases like "This experiment excludes theory Y at 95% confidence-level."
Sometimes scientists explicitly list which theories they are comparing in their experiment, and sometimes they simply mention the "null hypothesis." In any case, "evidence" is a set of observations that helps us to distinguish between multiple theories.