diff -r 2c6ebd0c850e -r de598e84c244 hgext/blackbox.py
--- a/hgext/blackbox.py	Wed Jan 10 21:27:05 2018 -0800
+++ b/hgext/blackbox.py	Sun Jan 14 12:06:27 2018 -0800
@@ -44,6 +44,7 @@
 from mercurial.node import hex
 
 from mercurial import (
+    encoding,
     registrar,
     ui as uimod,
     util,
@@ -182,7 +183,7 @@
                     fp.write(fmt % args)
             except (IOError, OSError) as err:
                 self.debug('warning: cannot write to blackbox.log: %s\n' %
-                           err.strerror)
+                           encoding.strtolocal(err.strerror))
                 # do not restore _bbinlog intentionally to avoid failed
                 # logging again
             else: