Is a reasonable level of knowledge of American Football advantageous when playing Blood Bowl?
Knowledge of American football strategy is not helpful in Blood Bowl because Blood Bowl lacks the rule that shapes all of American football play, namely, down and distance.  In American football you have four plays ("downs") to advance the ball 10 yards.  If you manage to do so, then you get another four plays to go another 10 yards (a "first down"), and so on until you either fail to make a first down in the allotted plays, or you score.  Almost everything about American football strategy revolves around managing down and distance.

Suppose, for example, that you make four yards on your first play, and five yards on the second. With only one yard to go you would choose a short-yardage play that highly likely to gain the one remaining yard, even though it has virtually no chance of making a large gain.  Getting tackled after two yards in that situation is a huge win.

Blood Bowl really has no equivalent.  Once a team fields the ball, they pretty much have to keep advancing it until they score.  If the ball carrier gets knocked down, you won't get an opportunity to reset for another play; you'll just recover the ball with another player and continue advancing, or you'll lose possession to the other team.  (I.e., every tackle effectively results in a fumble.)

Therefore, in a Blood Bowl game you'll never get much of a chance to apply anything you happen to know about American football strategy because you essentially never run a play from scrimmage, and that is what football strategy is all about.