diff --git a/tests/hghave.py b/tests/hghave.py
--- a/tests/hghave.py
+++ b/tests/hghave.py
@@ -140,9 +140,22 @@
     """Return the match object if cmd executes successfully and its output
     is matched by the supplied regular expression.
     """
+
+    # Tests on Windows have to fake USERPROFILE to point to the test area so
+    # that `~` is properly expanded on py3.8+.  However, some tools like black
+    # make calls that need the real USERPROFILE in order to run `foo --version`.
+    env = os.environ
+    if os.name == 'nt':
+        env = os.environ.copy()
+        env['USERPROFILE'] = env['REALUSERPROFILE']
+
     r = re.compile(regexp)
     p = subprocess.Popen(
-        cmd, shell=True, stdout=subprocess.PIPE, stderr=subprocess.STDOUT
+        cmd,
+        shell=True,
+        stdout=subprocess.PIPE,
+        stderr=subprocess.STDOUT,
+        env=env,
     )
     s = p.communicate()[0]
     ret = p.returncode
diff --git a/tests/run-tests.py b/tests/run-tests.py
--- a/tests/run-tests.py
+++ b/tests/run-tests.py
@@ -1371,6 +1371,7 @@
         env['TESTNAME'] = self.name
         env['HOME'] = _bytes2sys(self._testtmp)
         if os.name == 'nt':
+            env['REALUSERPROFILE'] = env['USERPROFILE']
             # py3.8+ ignores HOME: https://bugs.python.org/issue36264
             env['USERPROFILE'] = env['HOME']
         formated_timeout = _bytes2sys(b"%d" % default_defaults['timeout'][1])
diff --git a/tests/test-check-format.t b/tests/test-check-format.t
--- a/tests/test-check-format.t
+++ b/tests/test-check-format.t
@@ -1,5 +1,11 @@
 #require black test-repo
 
+Black needs the real USERPROFILE in order to run on Windows
+#if msys
+  $ USERPROFILE="$REALUSERPROFILE"
+  $ export USERPROFILE
+#endif
+
   $ cd $RUNTESTDIR/..
   $ black --check --diff `hg files 'set:(**.py + grep("^#!.*python")) - mercurial/thirdparty/**'`