If Meghan is proud to describe herself as biracial then its nobody elses business to tell her she should be 'more black'.
Nobody is saying she should be 'more white' are they?
Saying you are biracial is by definition not racist as it acknowledges you have 2 heritages. You really dont have to pick one to the exclusion of the other. In fact its pretty disrespectful to whichever parent you are rejecting. If 2 people loved each other enough to have kids without having a wobbly over each other's colour then they both deserve to be acknowledged in your heritage.
If you are mixed race you get to chose what feels right for you and only you. Nobody whatever colour thier skin is, gets to tell you who you should be.
The quicker people accept that people come in all colours not just ebony black or milk bottle white, have different heritages or criticise the way they do thier hair, the way they talk, what thier job is, how they live, who they know, the less we would need to keep going on about someone being 'not black enough'. What actually has a person to got do to prove they are 'black'?
A person has the right to determine all the above according to how they feel, thier own cultural preferences, thier environment, thier families not because someone says "you are x colour therefore you should do/believe/promote x,y,z to fit my expectations of you."