Ok I archived my mod and expected the BSA file... What is the BSL file ?

You can delete it along with the CKM. They're both interim files for uploading to Steam and seem to serve no other purpose. Deleting 'em won't cause any issues if updating something on the Workshop.

The BSL file contains data that the Archive.EXE application needs to properly read your BSA files (you received Archive.EXE with the CK download, it's used by the CK to package your resources). If you want to allow people to access your BSA with the Archive.EXE application, you'll need to include it in your download, but it's not essential as there are other tools that can read the BSAs without needing that file. Keep it on hand in case you want to make changes to the archive, though, otherwise if you open the BSA in the app, you'll just get gibberish.