If I had to use one word to sum up the art of blocking it would be "assists". These are incredibly important! Read the section on "Assisting A Block" in the Living Rulebook (page 21). If you watch a replay of two good bloodbowl coaches playing, you will probably see more than 50 assists in a single game.

In my opinion, it is usually far better to throw one block with two dice than two blocks with one dice. You have a better chance of knocking the opponent down, and there is far less risk of your player going down.

I'm not going to go into depth here, because it's covered fairly well in the Living Rulebook. Keep in mind though, that you won't learn everything about assists just from reading the book, you'll have to play a few games.