Hey mate,
Get hold of some dark brown, gray and black chalk pastels or better yet, powdered pigments, grind them up with some fine grit sandpaper and then dip a fine tipped brush into the powder. Lightly dab the area you want your blast marks to be and build up the colour gradually with repeated dabs of the brush. Blow away any excess with a turkey baster or an airbrush on low pressure setting. Just don't use your breath - the moisture in it can cause a mottled effect.
Chalks can be sealed with dullcote once you are happy with the results.
I would seal with dullcote beforehand though, the benefit of this is that the chalks will easily wipe off if you have made a mistake or been too heavy handed.
Hope this helps.