After importing your sequence of interest, click and drag within the Sequence Map to highlight the region you wish to BLAST. You can also use Command+A or Ctrl+A to select the entire sequence. Keep in mind that BLAST is limited to 5000 bp.
Once the sequence has been selected, click on the "Analyze" dropdown followed by the NCBI BLAST Button:
You can also right click on your sequence, and select NCBI BLAST from the drop down menu.
For non-academic customers:
If you'd like to have NCBI BLAST enabled for your Benchling account, please contact your Customer Success representative.