Q:
Is there a way to clean discolored blocks?
 Efflorescence
A:
There are concrete cleaning products available depending on the cause of the discoloration. Discoloration that appears as a white chalky powder is called efflorescence.
Efflorescence is a natural calcium carbonate discharge on the surface of concrete products. It is caused by the products used to create the block and it can disappear over time. Cleaning products can help to eliminate the problem, but cleaning is not crucial to the life of your block.
How do you clean leaf stains off allan block, what type of sealer should you put on it to eliminate further stains?
To clean stains off a block you will want to use a concrete cleaner. Your local hardscapes store should be able to recommend one that will do the job. As for a sealer, there are many different types on the market and will provide a variety of finishes to the wall. For example some have a clear finish while others leave a shine – like a “wet look” which can alter the color of your wall. Keep in mind once you apply a sealer to your wall, you will need to reapply as needed to keep the wall looking fresh and clean. I would recommend simply cleaning the stains as they appear rather than sealing the wall. However, If you really want to seal the wall, talk to your local hardscape supplier for a recommendation on a sealer available in your area.