@Dastardlythefriendlymutt I understand that topic is a minefield! I'll do my best to give my perspective.
You're absolutely right that I as a mixed race person can identify as black, or mixed, but not as white. I am fairly pale, which on a side note; MM is not white passing at all in my opinion. I saw that suggestion yesterday on another forum, but I was in disbelief. The moment I saw her in Suits I knew she was mixed race.
I can absolutely see where you're coming from. My mother is 'black black', I'm 'mixed black'. DM has made it clear my entire life that she believes I have a greater advantage in life than her. I can see why she thinks that, but my experiences haven't been as plain sailing as she would have thought.
It's a multi faceted issue. Some people have very strict 'criteria' for what passes as white, and any kind of mix will instantly exclude you. Often times, those same people are the ones with racist views, but I have genuinely met some that didn't have a racist bone in their body but they just saw it as a fact. I've never wanted to pass as white, so I've never cared, but it has always been interesting that I've been black even though I'm only 50% black, but I can't be white when I am 50% white. To me, it feels like the distinction between a well known brand and other competitors. A Hoover is actually a brand, vacuum cleaners are often called Hoovers but they're not. They can never be Hoovers, but a Hoover is always a vacuum cleaner. I don't know if that even makes any sense 
Anyway, I do actually get the same treatment from people who are fully black, or much darker than me. They have had struggles that they don't believe I could understand and therefore they don't think I'm entitled to class myself as one of them. At one point I was bullied and called all sorts of names by black girls at my school, including 'Albino'. (Which by the way is one of my favourite examples of a racist incident that I do not consider to be anywhere near as bad as the racism towards black people in the UK). I think if I was to enter a competition for black writers, say, and won; people would be upset. I understand that, and I don't put myself forwards for things like that, because I have a deep understanding that I could probably have a better chance of winning a prize in a regular competition than black people with darker skin. They need to be shown that they are important and they need opportunities that normal life doesn't hand out to them so freely. They need equity. I think this is what you meant about the disadvantages of blurring the lines, and I think it's important that some mixed people realise that.
But the main thing for me as a mixed person who is proud of my black heritage is opportunities like these on MN, to talk to people who know what I mean when I say steups or ask where to buy breadfruit. Or could give me advice on my hair. Those are the times when I would love the shade of my skin not to matter, because I feel just as black as anybody else.