Let’s Play A Game 

*kisses you* Thank you, I love you.

Martha is my second-favourite New Who companion (after Donna, obviously), so they were certainly doing something right. But  issue is the way the Doctor treats her. He ignores the fact that her race may be a problem in Elizabethan England (if I was in England any time before the 80s I’d be terrified) and strands her in pre-war England as a maid. At least in the Family of Blood episode her race is mentioned and addressed (brilliantly, I might add). But the Doctor’s ignorance of the problems that could arise whilst travelling back in Europe’s history with a black woman is baffling. Also, the pairing with Martha and Mickey at the end of the End of Time is addressed by Feminist Whoniverse: 

Their scene in The End of Time could have featured them working together as close friends with enough sexual tension and romantic subtext to hint at the possibility of a future relationship, but that didn’t happen. 

The fact is that Martha and Mickey suddenly appear as a couple and the only reason for this pairing that can be clearly seen by the audience is that both characters are black.

The fandom’s reaction to her is covered by this brilliant little post by tenlittlebullets, but I hope that answers your question :) And remember, this is only my perspective, so talking to other Black Whovians about this is a must.