It's a best practice to compare the reports of two or more sessions of the same test. With BlazeMeter it's easy to do this:
1. Click the 'Compare' button in the top right corner of the Reports section when you are viewing a certain report.
2. Add the report or reports that you want to compare from the drop-down list or simply drag and drop them. You can add a maximum of five reports.
3. You will be shown a different graph for each KPI comparison between the different reports.