Nah, you're absolutely right. It's always been an obstacle in her career. I like to think that the market is much more open than it was years ago which is why she's doing better than the slump she went through several years ago. It's good for her career-wise but a bit boring for me music-wise. The only important thing is that she builds a strong following which will remain to the day where she won't be getting as many tie-ins anymore because like you said, there are many other who do the same thing; anyone could sing drama songs.
But she was very successful before the slump, much more so than she is now. I doubt Japan just decided to discriminate against her all of the sudden when she was scoring Top 10 singles and hit albums for half a decade.
We have been through this before when VIVID flopped. CK has enjoyed a long career not just as a Black artist in Asia but as a J-pop singer in general. She was popular, then she wasn't, now she's trying to build herself back up. It's natural for someone's careers to have ups and downs. I'm sure Crystal has faced some obstacles due to her mixed background. There are probably things that she'll never be asked to do because she doesn't look traditionally Asian (like a make-up commercial), but she has prospered in her career despite that. She's done good. Her fans just need to accept that she is just not that popular right now and it may not have anything to do with her background. She just needs to find something that works for her and the public. And really that's every artist.