How to download a file from rails application
Don't use send_file with a parameter set by a user. This opens up a massive security hole, allowing a user to access any file that is readable by your application (namely, your entire application, but also possibly other files on the filesystem).
Rather, if the file is under public, link to the file itself. In your case:
<%= link_to "Raw blast output", "/data/02_blastout/#{@bl_file}" %>No need for a special controller action.