Thank you for submitting what is an AWESOME question !! The answer is actually fairly straightforward, but it is a question that everyone has when they first hear/learn about write blockers and how they work.
Take a look at the url below, as it will help you out:
Write Blockers - ForensicsWiki
Write blockers are devices that allow acquisition of information on a drive without creating the possibility of accidentally damaging the drive contents.
The short answer to the question is that the write blocker is designed to prevent write commands from being issued back through the interface while allowing read commands through to facilitate access to the target data, but to do so without the ability to modify any attributes of the data in any way.