--- a/hgext/blackbox.py	Wed Sep 06 21:23:38 2017 -0700
+++ b/hgext/blackbox.py	Mon Sep 18 16:01:03 2017 -0700
@@ -192,6 +192,13 @@
 
     if util.safehasattr(ui, 'setrepo'):
         ui.setrepo(repo)
+
+        # Set lastui even if ui.log is not called. This gives blackbox a
+        # fallback place to log.
+        global lastui
+        if lastui is None:
+            lastui = ui
+
     repo._wlockfreeprefix.add('blackbox.log')
 
 @command('^blackbox',