diff --git a/mitogen/core.py b/mitogen/core.py
index 836c451a..d2d20723 100644
--- a/mitogen/core.py
+++ b/mitogen/core.py
@@ -168,6 +168,14 @@ def restart(func, *args):
 
 
 def is_blacklisted_import(importer, fullname):
+    """Return ``True`` if `fullname` is part of a blacklisted package, or if
+    any packages have been whitelisted and `fullname` is not part of one.
+
+    NB:
+      - If a package is on both lists, then it is treated as blacklisted.
+      - If any package is whitelisted, then all non-whitelisted packages are
+        treated as blacklisted.
+    """
     return ((not any(fullname.startswith(s) for s in importer.whitelist)) or
                  (any(fullname.startswith(s) for s in importer.blacklist)))
 
@@ -412,7 +420,7 @@ class Importer(object):
             'utils',
         ]}
         self._lock = threading.Lock()
-        self.whitelist = whitelist or ['']
+        self.whitelist = list(whitelist) or ['']
         self.blacklist = list(blacklist) + [
             # 2.x generates needless imports for 'builtins', while 3.x does the
             # same for '__builtin__'. The correct one is built-in, the other
diff --git a/tests/importer_test.py b/tests/importer_test.py
index 5fe65f45..55cad182 100644
--- a/tests/importer_test.py
+++ b/tests/importer_test.py
@@ -9,6 +9,8 @@ import pytest
 import unittest2
 
 import mitogen.core
+import mitogen.utils
+
 import testlib
 
 
@@ -25,6 +27,63 @@ class ImporterMixin(testlib.RouterMixin):
         super(ImporterMixin, self).tearDown()
 
 
+class ImporterBlacklist(testlib.TestCase):
+    def test_is_blacklisted_import_default(self):
+        importer = mitogen.core.Importer(
+            router=mock.Mock(), context=None, core_src='',
+        )
+        self.assertIsInstance(importer.whitelist, list)
+        self.assertIsInstance(importer.blacklist, list)
+        self.assertFalse(mitogen.core.is_blacklisted_import(importer, 'mypkg'))
+        self.assertFalse(mitogen.core.is_blacklisted_import(importer, 'mypkg.mod'))
+        self.assertFalse(mitogen.core.is_blacklisted_import(importer, 'otherpkg'))
+        self.assertFalse(mitogen.core.is_blacklisted_import(importer, 'otherpkg.mod'))
+        self.assertTrue(mitogen.core.is_blacklisted_import(importer, '__builtin__'))
+        self.assertTrue(mitogen.core.is_blacklisted_import(importer, 'builtins'))
+
+    def test_is_blacklisted_import_just_whitelist(self):
+        importer = mitogen.core.Importer(
+            router=mock.Mock(), context=None, core_src='',
+            whitelist=('mypkg',),
+        )
+        self.assertIsInstance(importer.whitelist, list)
+        self.assertIsInstance(importer.blacklist, list)
+        self.assertFalse(mitogen.core.is_blacklisted_import(importer, 'mypkg'))
+        self.assertFalse(mitogen.core.is_blacklisted_import(importer, 'mypkg.mod'))
+        self.assertTrue(mitogen.core.is_blacklisted_import(importer, 'otherpkg'))
+        self.assertTrue(mitogen.core.is_blacklisted_import(importer, 'otherpkg.mod'))
+        self.assertTrue(mitogen.core.is_blacklisted_import(importer, '__builtin__'))
+        self.assertTrue(mitogen.core.is_blacklisted_import(importer, 'builtins'))
+
+    def test_is_blacklisted_import_just_blacklist(self):
+        importer = mitogen.core.Importer(
+            router=mock.Mock(), context=None, core_src='',
+            blacklist=('mypkg',),
+        )
+        self.assertIsInstance(importer.whitelist, list)
+        self.assertIsInstance(importer.blacklist, list)
+        self.assertTrue(mitogen.core.is_blacklisted_import(importer, 'mypkg'))
+        self.assertTrue(mitogen.core.is_blacklisted_import(importer, 'mypkg.mod'))
+        self.assertFalse(mitogen.core.is_blacklisted_import(importer, 'otherpkg'))
+        self.assertFalse(mitogen.core.is_blacklisted_import(importer, 'otherpkg.mod'))
+        self.assertTrue(mitogen.core.is_blacklisted_import(importer, '__builtin__'))
+        self.assertTrue(mitogen.core.is_blacklisted_import(importer, 'builtins'))
+
+    def test_is_blacklisted_import_whitelist_and_blacklist(self):
+        importer = mitogen.core.Importer(
+            router=mock.Mock(), context=None, core_src='',
+            whitelist=('mypkg',), blacklist=('mypkg',),
+        )
+        self.assertIsInstance(importer.whitelist, list)
+        self.assertIsInstance(importer.blacklist, list)
+        self.assertTrue(mitogen.core.is_blacklisted_import(importer, 'mypkg'))
+        self.assertTrue(mitogen.core.is_blacklisted_import(importer, 'mypkg.mod'))
+        self.assertTrue(mitogen.core.is_blacklisted_import(importer, 'otherpkg'))
+        self.assertTrue(mitogen.core.is_blacklisted_import(importer, 'otherpkg.mod'))
+        self.assertTrue(mitogen.core.is_blacklisted_import(importer, '__builtin__'))
+        self.assertTrue(mitogen.core.is_blacklisted_import(importer, 'builtins'))
+
+
 class LoadModuleTest(ImporterMixin, testlib.TestCase):
     data = zlib.compress("data = 1\n\n")
     path = 'fake_module.py'