Mold is common in attics. If you look at the black mold, you should be able to tell whether it was on the wood to begin with or grew after the house was built. If the rafters have black colonies on them, the wood was most likely contaminated to begin with.
According to The Money Pit's indoor air quality expert, Jeff May, in cold climates, if the sheathing is darkest on the north- or east-facing gable (worse near the outer, lower part of the sheathing), or if the attic mold appears only at the top of the rafters, the attic mold may be growing due to excess moisture, most often caused by improperly vented bathroom exhausts and leaky attic accesses that allow warm, moist house air into the attic.
But if you see oval- or other-shaped colonies that appear to be covered and bisected by rafters, the sheathing may have been moldy when installed.
For more tips on black mold and mold proofing your home, see our Mold Resource Guide.