Not a bad idea, but as someone else said, using a grey spray primer is just easier. I used the Tamya grey spray once, and it's rather good ; I think the color you use for primer depends mostly on the colors you will use on your mini and the way you like to paint. For instance, you'll use white primer if you want to have brigh colors, don't like dry brushing and prefer wet blending..or just if you want to paint your mini in yellow (those who have try to paint yellow over black primer still are screaming in pain).
If you have a knight in armor and don't like the trendy NMM style, go for black primer. Anyway, go for black if your mini has mor metal than clothes.