Block in the Back downfield

[mulligan23]

I had an instance in an 8th grade game where I disagreed with the Referee, and the Umpire.

Situation: Ball spotted on the 50 yard line, Team A (offense) runs the ball to about the 15 yard line, on the way down the field a block in the back was called by the umpire the covering official at the 20. I assumed the block in the back would be penalized from the 20 back 10 yars, and then the result would still be a first down? Am I incorrect, as the penalty was administered from the 50 and took the team back to their 40 yard line. I read my books, and feel that I was correct, should penalize from the spot of the foul.