When running your Tests in Blazemeter, using JSR223 Pre/Post processors is a common practice to extract desired data from a response to specific file. Thus, it's very important to know that when writing a file using this element, you will need to include the following path to ensure the file gets written to the correct folder:
i.e.: /home/jmeter/Results.txt (or .CSV)
As a result, you'll find the file in the Logs report inside the "jtls_and_more.zip" file of your Test results.
NOTE: There's a limitation in BlazeMeter where writing files in this architecture where you will need to run a "Console only" JMeter test which means that you need to use the 'Users' slider, while not exceeding 500 users. If you need more "Users" than this, then, you will need to create a Multi test with the number of console tests needed to reach your desired load.