Detroit's are known for this: if one runs away, replace the shaft seals or the next time it runs away it might not shut down because of oil leakage from the last run away. (seen in cumminstinkerer's post)
You said it yourself: when the inlet to the blower is plugged, the blower can't blow.
Since the engine is still trying to pull air at high RPM's but the intake is blocked, a vacuum is created (like a choke in a gas engine), so the engine pulls oil and air through the weakened blower seals.
I hope this explanation makes sense! ๐