Use black paint on its own, otherwise you will need to keep adding black until the grey becomes black enough. Adding a very dark blue to grey will make a black, but it won't be a proper black, more like a bluey black.
To make gray, you can mix black and white paint together. Start by adding a small amount of black to white until you achieve the desired shade of gray.
black and white make gray
gray
Black and White
Black and White
gray
No the standard colors are black, gray and white.
Mix colors black and white together to get gray!
Black mixed with grey
Black and white.
white and black
Gray is made by mixing equal parts of black and white. The more black you add, the darker the shade of gray.