Inspired by the end of this answer:
Just remember that all non-Jews don't follow the Sheva Mitzvot and could [even] be in Avodah Zarah and then would...a blessing [of theirs] matter? It might even be harmful (emphasis mine)?
Is the latter notion above correct? If someone prays for another's well-being, and the prayers are not (entirely) directed towards HaShem, could that harm the person whose well-being is being prayed for?
Bar Hedya said to himself: What am I to do? We have been taught that a curse uttered by a sage, even when undeserved, comes to pass; how much more so of Rava's, which I deserved!