Using the diagnosis "blown out" knee is not terribly helpful in understanding exactly what structures were damaged nor what the injury prognosis will be. Saying a patient "blows out" their knee is much like saying the patient sustained a "knee sprain". Neither of these terms is a medical diagnosis.
The proper way to describe this condition is by stating which knee ligaments were damaged at the time of the injury. In the case of Daunte Culpepper's injury, he sustained damage to three of the four major ligaments in the knee; the anterior cruciate ligament, the posterior cruciate ligament, and the medial collateral ligament. This extent of damage to the knee and the treatment that will be required will certainly end the football season of this athlete.