I don't know how long they last, but I've seen several relationships like the type you described. 

I have seen some black men with Chinese women, and a couple of them have worked out very well.

However I have not seen any  black women with Chinese men (and I am  in my 6th year here).

all you need to do is take a short trip to guangzhou, you will see many black guys with chinese gf,, as for working out im not sure, the few black guys i know are doing well but i dont know that many black people in guangzhou, i live far from those communities 

They do exist. I don't know how common it is. Here is a Chinasmack article: http://www.chinasmack.com/2011/pictures/chinese-men-with-black-women-african-wives.html

In almost 3 years, I can count the number of times I've seen a black woman in China on one hand, never hand in hand with a Chinese man.  Maybe it's different in the big three: BJ, SH, GZ....

You'll see more of this as China colonizes the African continent. I'm not against intermarriage.

black to Chinese relationships does not exist here unless the Chinese has traveled to Africa and met black woman there,like getting hooked by Chinese here is ridiculous but in Africa it happens coz they don't know what it means to be in a relationship with Chinese.Those who have managed to come in china from Africa with their lovers,they return home next day.