Naturally, I agree with you that the acts you describe are evil. I think you're misunderstanding moral relativism here. The point is not that we, who belong to roughly the same culture, can't consider anything to be evil or good. The point is that the people belonging to the other cultures may not feel the same way about it.
As it happens, no cultures ritually blind their children, but some do in fact do some pretty horrendous and pointless things. We will find that in each such case, religion is the reason.
Jewish parents in Jewish community do not consider themselves evil because they have their male children circumcised, even though they are participating in a painful and completely unnecessary surgical procedure without anesthesia. Worse than this is the similar custom of female infibulation that is found in some Muslim countries, but the parents do not think they are doing an evil thing. Exceptions, in both cases, come only when there has been exposure to our attitudes toward these customs, generally through media or aid workers.
The people who are subject to those religions, customs, and laws are not evil people because they willingly comply with them. We may think of the particular acts as being intrinsically evil. I certainly do. However, I don't quite feel self-righteous enough to believe that the people performing them are evil. I believe they do what they have been taught is right by their culture.
So, objectively, we are just as right as the people who mutilate the genitals of their infants. Subjectively, however, we are right and they are wrong. Or evil, if you prefer.
Moral absolutism is a sure path to war, which itself could be considered evil.