Thank you for your question! Please refer to my answer to Mariela regarding how we can meaningfully face the culture in general.
Concerning the BL comics and other J-pop things, it depends on what you mean by “harmful” or “harmless,” but I think it is difficult to make a case from a Christian perspective that they do not influence Christian life at all. It seems that some BL comics are sexually explicit, and I won’t encourage Christians to read them in the same way I won’t encourage them to watch porn. And when it comes to comics that are not necessarily sexually explicit, we still need to be aware of the fact that anything produced by human beings has some kind of underlying message behind it. Comics that portray sexual relationships outside of marriage (e.g., having pre-marital sex or an affair) in a positive way either explicitly or implicitly tell us that they are fun and need to be celebrated. BL comics are no exception, and that is why we need to be aware of how the culture influences us.
Traditionally, Japanese culture seems to be somewhat conservative on sexual morality, but just like many other Asian countries, we often follow the trend of western cultures, which is gradually making them open to celebrating various ideas on sexuality. But I would also like to point out that even if a certain culture has a moral value that is apparently in line with the Christian perspective, that does not necessarily mean they are completely righteous before God. For example, many people I meet outside of Japan tell me how they are impressed by how Japanese people behave well in public, such as keeping places tidy and clean or refraining from being noisy in public transports. Yes, those things are good, but often time they do it because of the peer pressure or the fear of being ashamed. For many people in Japan, it appears that their idol in life is the approval of other people, and doing good things to pursue it is not a good enough motive. We are to do good things out of love for God and love for our neighbors. That is why all cultures fundamentally need the gospel of Christ, and we need to pray for Japan regardless of how the secular people live “moral lives.”