For successful shading and blending, I've used an ordinary 0.5mm or 0.7mm mechanical pencil loaded with 2B since I started drawing... this gives me a fine line at all times without the need to constantly sharpen. These pencils are usually sold with HB leads, so be sure to purchase a pack of 2B to replace them. Unlike the wooden pencil, you can't use the side of the lead to shade large areas, instead, the shading is kept very tight and the varying tones are achieved by gradation and layering, with the pencil strokes always following the shape of your object (unlike cross hatching). I find that smooth, dark areas are best achieved by alternate layering and blending rather than extra pressure. I may apply 5 - 6 (or more) layers of pencil to an area of dark shadow or hair.
Because of scanning limitations, I've drawn quite heavy to demonstrate this method but of course your initial outline will be much lighter. The hair is no more difficult to draw than any other facial feature, although it can take considerable time to achieve a natural look. I've actually drawn in where the shades differ and indicated the direction of growth of each section.
The first thing I do is shade in all the darkest tones drawing in the direction the hair lies. Closing the eyes slightly and squinting at your reference photo will help to define these areas. How dark I shade these areas is obviously dependent on the colour of the hair. A blonde colour would require a much lighter shade but the actual method would be identical.
Once shaded, I'm going to use the tortillon to drag the colour into the the light areas, again in the direction the hair lies, until I achieve a halftone (mid gray). I then open up the highlights again by dragging my eraser from the light areas, into the dark.
I tend to hold the pencil quite high and use use long, sweeping flicks of the pencil, very lightly, from the dark areas... into and across the light areas to represent the individual hairs. Needless to say, these strokes need to follow the hair growth. Blend this layer as before, again, using the tortillon from dark to light and the eraser from light to dark to re-establish the highlights.
Erase lightly so that the previous layers show through, this gives the impression of depth to the hair. Notice how the lighter areas of these drawings change shape slightly, this is due to erasing to re-establish the highlights. The drawing on the left has just the two layers, I would probably do a third or even a fourth layer to give a more luxuriant look. Always finish by softening the outside edges with the blender.