The large black area is organic growth, most likely mould, caused by excessive attic humidity. Sections flake away like old paint chips as the lifecycle continues.
Section 9.19 of the National Building Code of Canada sets the requirements venting attics. For roofs with a slope of 1:6, and greater, the unobstructed vent opening must be at least 1/300th of the insulated ceiling area, with not less than 25% of the opening at the top, or at the bottom.
Every joist cavity must be vented using baffles, purlins, or a minimum 63 mm clear space below the underside of the roof.