First install the latest, stable version (currently 2.10.2) of the Open-Source AADL Tool Environment (OSATE) following the instructions at

The BLESS plugin can be installed into OSATE by Help -> Install Additional OSATE Components.

Check the box beside:

BLESS Editor and Proof Assistant

Click Finish. Read, then accept, the licenses. Finish, again.

OSATE will ask whether to restart. Do so.