When you combine grey and black, the result will be a darker shade of grey. This is because grey is already a mixture of black and white, and adding more black to the mixture will make it darker. The exact shade of grey that you get will depend on the ratio of black to grey in your mixture. If you add a small amount of black to a large amount of grey, you will get a slightly darker shade of grey. If you add a large amount of black to a small amount of grey, you will get a much darker shade of grey.
It's worth noting that black is a neutral color, meaning it doesn't have any hue. When you mix black with a color, it will make that color appear darker, but it won't change the color's hue. Grey is also a neutral color, but it has a slight hue of its own. This means that when you mix grey and black, the resulting color will still have a slight hue, but it will be much less noticeable than the hue of the original grey. In general, the more black you add to grey, the less noticeable the hue will be. This is why black and grey are often used together in interior design to create a monochromatic, moody look.