Someone recently asked me why it was important to block loops quick off the bounce. He thought that taking the ball quick off the bounce made it harder for the blocker to react, and made the shot more predictable for the opponent. However, it's actually easier and more consistent to block a loop off the bounce, and the lack of variation from this only happens if the blocker doesn't vary his block.
You do want to block loops off the bounce. If you take it late, you have more ground to cover (often with little time to react, depending on the speed of the loop), as well as having to predict the ball's fast and low bounce off the table. Even more important, blocking quick off the bounce allows you to both rush and angle an opponent. If you take the ball late, your opponent has time to react to your shot, and the block loses its effectiveness.
So how do you make your block more effective by varying it? First of all, while variation is important, consistency is most important. Your block needs to be a steady, quick over-the-table shot - think of yourself as a wall. But you can be steady and still mess up your opponent by varying your blocks.
So, how can you vary your block to mess up an opponent? Here's a rundown.
Steadiness: Sheer consistency, combined with quickness, will wear down an opponent. If you combine steadiness with at least one other variation, such as placement or change of pace, your block will be even more effective.
Three placements: Your blocks should almost always go to one of the three main placements - wide forehand, wide backhand, and middle. By blocking quick off the bounce, your opponent has little time to react to these, and they all force him out of position.
Deceptive placement: You can aim one way, and at the last second change directions. For example, suppose you aim your backhand block to the opponent's wide backhand (for two righties). At the last second, just bring your wrist back and block down the line to the forehand. Deceptive placement is perhaps the most under-used tactic in blocking at the intermediate level - far too often players telegraph where they are blocking early in the shot.
Tactical placement: Sometimes you should hammer an opponent's weak side over and over. Or you might want to go after the opponent's middle over and over. Against others, you might want to go quick to their strong side to draw the opponent out of position, and then come back to the weak side, making them move and hit their weaker shot.
Speed: Not all blocks are passive. A jab block is an aggressive block, and if placed well - see three placements above - is particularly effective. You can especially jab block against a loop that lands short. Slow, spinny loops that land short are easy to miss if you take them late, but if you take them off the bounce decisively, they are easy to jab block or even smash. To smash them, shorten your backswing.
Dead block: These are great to vary the pace and throw off an opponent's timing, and are especially effective when combined with a jab block, or against an opponent who tends to back off the table. Just hold the racket loosely and let the ball rebound out slowly.
Chop block: If you chop down on the ball at contact, you can dead block with backspin. This is more easily done on the backhand.
Sidespin block: At contact, move the racket sideways to create sidespin. You can do this in either direction. This is another way to dead block. This is more easily done on the backhand.
Topspin block: You can topspin the ball right off the bounce, both backhand or forehand, sort of a mini-loop. (Some call this the "kiss of topspin.") At the world-class level this is the most common type of block.
Use block to set up attack: Blocking by itself will only take you so far. If you use your block to mess up an opponent so that you can then attack yourself, you put even more pressure on your opponent. Any time you sense your opponent will have trouble with your block, get ready to take the attack.
Many or most of the above may be tricky to do at first, but that's because you haven't been doing them. Decide which variations above best fit your game. Then find time to practice them in drills, use them in practice matches, and soon they'll become second nature.