Bless Hex Editor only showing ASCII squares
Converting my comment and @user.dz's comment to an answer as requested.
As per this bug report, the issue is caused because the latest version of an upstream library (pango) deprecated the use of certain fonts, causing breaking changes to older programs which used these fonts. This mainly affects users on Ubuntu 20.04, which uses the upgraded pango libraries and the 0.6.0 bless version.
The bug has been fixed in the latest bless releases on github.