How does shading & blending with Carbon pencils differ from Graphite pencils?
I find that Carbon Pencils tend to dry out and break easily while I work. If I use carbon, I try buy one which is oil based; they tend to have strong tonal values without the shininess of graphite. You can make a very sharp line with a neatly sharpened pencil. I sometimes wet it with some linseed oil, or saliva, if it looks a bit dry. 

Graphite is used as a lubricant in some industrial processes. So it applies more smoothly but I find the shine is annoying at times. You should consider the grade of your graphite as well. The ratio of graphite mixed with clay will affect the shininess in some pencil brands. The Staedtler Black Red 7B is an example that doesn't have a shiny gloss compared to other brands.