diff --git a/hgext3rd/sampling.py b/hgext3rd/sampling.py
--- a/hgext3rd/sampling.py
+++ b/hgext3rd/sampling.py
@@ -73,6 +73,13 @@
              "msg":"",
              "opts":{"t":42}}\0
             """
+            # special case: remove less interesting blocked fields starting
+            # with "unknown_" or "alias_".
+            if event == 'uiblocked':
+                opts = {k: v
+                        for k, v in opts.items()
+                        if (not k.startswith('alias_') and not
+                            k.startswith('unknown_'))}
             if not util.safehasattr(self, 'samplingfilters'):
                 self.samplingfilters = logtofile.computesamplingfilters(self)
             if event not in self.samplingfilters: