This is the process. I told myself how would I proper render blond or red body hair on black & white. Actually you see here I used a very light value for the graphite body hair. And just as Patrick suggested, I used a red colored pencil to reshape this light grey hair, before performing the shading. The mix doesn't seem bad to me, just a bit odd, haha!
I just love the swirling pattern of Matthieu's chest hair! So fun to draw (and long, as any could imagine!) :-)