The fact that the pass doesn't get off doesn't have any effect at all.
It comes down to the judgement of the official. Your DB's are able to BLOCK all they want to... tackling is, technically illegal, but usually if the blitz is successful, all heck breaks loose, and there won't be any holding. Consider this, though... suppose the QB eludes the blitz and rolls out to his right, the receiver has been tackled as he was trying to run an out pattern... that is defensive holding, and is illegal. Now, if the receiver did, indeed approach the DB in a "blocking manner," they may legally ward him off. In High School, you don't see it called much because there are only 5 officials on the field. Usually the only time you'll see it is when a corner gets beat off the line, and grabs the receiver's jersey as he goes past.
If you have a good crew of officials, and they see your DB's tackling receivers, don't be surprised if it's called.
But, the original play, as it was described (whether there was holding or not, I sure don't know) the administration of the penalty was correct.