I used to use black primers... have since gone completely off them in favor of grey.
I find grey gives a more vibrant colour when you base coat over it. And is easier going when working upto white or similarly light colours.
The only plus point I can think of for black was that when you cant reach a place with a brush, its a dark (black) shadow so you get away with it.
When using grey, I have had to do more things separate to the model, but I feel that the painting quality went up for it.
Just painting caliban green over grey primer, isnt going to look the same as painting it over black primer, so if you need it to match up with the rest, go with black

save the grey can for something else, maybe the deathwing to save on working up from black to almost white.