diff --git a/src/pynguin/analyses/module.py b/src/pynguin/analyses/module.py
index 4be96e199..f974b1edf 100644
--- a/src/pynguin/analyses/module.py
+++ b/src/pynguin/analyses/module.py
@@ -416,6 +416,21 @@ def _is_blacklisted(element: Any) -> bool:
 ))
 
 
+def public_top_level_module(module_name: str) -> str:
+    """Return the top-level module of a module name without leading underscores.
+
+    C implementations of standard-library modules often report a private module,
+    e.g., ``io.TextIOWrapper.__module__`` is ``"_io"``.
+
+    Args:
+        module_name: The (possibly dotted) module name.
+
+    Returns:
+        The top-level module name without leading underscores.
+    """
+    return module_name.split(".", maxsplit=1)[0].lstrip("_")
+
+
 def _c_is_whitelisted(element: ModuleType | str) -> bool:
     """Checks if the given element belongs to the C module whitelist.
 
@@ -429,9 +444,7 @@ def _c_is_whitelisted(element: ModuleType | str) -> bool:
 
     try:
         module_name = element if isinstance(element, str) else element.__name__
-        top_level = module_name.split(".")[0]
-        public_top_level = top_level.lstrip("_")
-        return public_top_level in c_module_whitelist
+        return public_top_level_module(module_name) in c_module_whitelist
     except Exception:  # noqa: BLE001
         LOGGER.warning(
             "Could not check if %s is whitelisted. Assuming it is not.", element, exc_info=True
diff --git a/src/pynguin/assertion/assertiontraceobserver.py b/src/pynguin/assertion/assertiontraceobserver.py
index 66e008c86..fb55d7715 100644
--- a/src/pynguin/assertion/assertiontraceobserver.py
+++ b/src/pynguin/assertion/assertiontraceobserver.py
@@ -26,7 +26,7 @@
 import pynguin.configuration as config
 import pynguin.testcase.execution as ex
 import pynguin.utils.typetracing as tt
-from pynguin.analyses.module import MODULE_BLACKLIST
+from pynguin.analyses.module import MODULE_BLACKLIST, public_top_level_module
 from pynguin.assertion.assertion_to_ast import assertion_to_cst
 from pynguin.utils.exceptions import TracingAbortedException
 from pynguin.utils.naming import get_module_alias
@@ -105,7 +105,13 @@ def _is_blacklisted_module(module_name: str | None) -> bool:
         return False
     blacklist = set(MODULE_BLACKLIST).union(config.configuration.ignore_modules)
     blacklist.discard("builtins")
-    return module_name in blacklist or module_name.split(".")[0] in blacklist
+    # Also match C implementations, e.g., "_io" for "io", next to private entries
+    # of the blacklist itself, e.g., "_thread".
+    return (
+        module_name in blacklist
+        or module_name.split(".")[0] in blacklist
+        or public_top_level_module(module_name) in blacklist
+    )
 
 
 def _is_blacklisted_value(value: Any) -> bool:
diff --git a/tests/analyses/test_native_libraries.py b/tests/analyses/test_native_libraries.py
index cbd0a8e16..28a58be26 100644
--- a/tests/analyses/test_native_libraries.py
+++ b/tests/analyses/test_native_libraries.py
@@ -48,6 +48,19 @@ def test_c_is_whitelisted():
     assert not pm._c_is_whitelisted(fake_mod)
 
 
+@pytest.mark.parametrize(
+    "module_name,expected",
+    [
+        ("_io", "io"),
+        ("_json.decoder", "json"),
+        ("json", "json"),
+        ("numpy.linalg", "numpy"),
+    ],
+)
+def test_public_top_level_module(module_name, expected):
+    assert pm.public_top_level_module(module_name) == expected
+
+
 def test_c_is_whitelisted_exception():
     """Verify that non-module or broken objects return False without crashing."""
 
diff --git a/tests/assertion/test_assertion_generation_integration.py b/tests/assertion/test_assertion_generation_integration.py
index 749f33a3e..0fad5e24f 100644
--- a/tests/assertion/test_assertion_generation_integration.py
+++ b/tests/assertion/test_assertion_generation_integration.py
@@ -1170,3 +1170,31 @@ def test_mutation_analysis_integration_package_with_relative_import(
         assert num_created > 0
         assert metrics.num_created_mutants == num_created
         assert metrics.num_killed_mutants > 0
+
+
+def _tc_std_streams(alias: str) -> tc.TestCase:
+    return make_test_case(
+        call_stmt("stream_holder_0", f"{alias}.StreamHolder()"),
+        call_stmt("shlex_0", f"{alias}.create_lexer()"),
+    )
+
+
+def test_no_assertions_on_std_streams(subject_properties: SubjectProperties):
+    # Stream objects such as sys.stdin and sys.stdin.buffer are io types from the "_io"
+    # module, and pytest's capture replaces them, so assertions on them do not hold there.
+    module_name = "tests.fixtures.examples.std_streams"
+    config.configuration.module_name = module_name
+    alias = get_module_alias(module_name)
+    with install_import_hook(module_name, subject_properties):
+        with subject_properties.instrumentation_tracer:
+            module = importlib.import_module(module_name)
+            importlib.reload(module)
+
+        test_case = _tc_std_streams(alias)
+        _suite(test_case).accept(ag.AssertionGenerator(TestCaseExecutor(subject_properties)))
+
+    source = _render(test_case)
+    assert "assert stream_holder_0.name == 'stdin'" in source
+    assert "_io." not in source
+    for stream_field in ("stream_holder_0.text", "stream_holder_0.binary", "shlex_0.instream"):
+        assert stream_field not in source
diff --git a/tests/assertion/test_assertiontraceobserver.py b/tests/assertion/test_assertiontraceobserver.py
index 5d2e40b12..45d1ee758 100644
--- a/tests/assertion/test_assertiontraceobserver.py
+++ b/tests/assertion/test_assertiontraceobserver.py
@@ -19,6 +19,7 @@
 from __future__ import annotations
 import __future__
 
+import io
 import logging
 import os
 import sys
@@ -708,6 +709,37 @@ def test_is_unstable_runtime_value():
     assert ato._is_unstable_runtime_value(list(sys.path)) is False
 
 
+@pytest.mark.parametrize(
+    "module_name,blacklisted",
+    [
+        ("_io", True),
+        ("_asyncio", True),
+        ("_csv", True),
+        ("_thread", True),
+        ("_iox", False),
+        ("_pytest", False),
+    ],
+)
+def test_is_blacklisted_module_for_private_modules(module_name, blacklisted):
+    # C implementations report private modules, e.g., "_io" for the blacklisted "io".
+    assert ato._is_blacklisted_module(module_name) is blacklisted
+
+
+@pytest.mark.parametrize(
+    "stream",
+    [
+        io.TextIOWrapper(io.BytesIO(), encoding="utf-8"),
+        io.BufferedReader(io.BytesIO()),
+        io.StringIO(),
+        io.BytesIO(),
+    ],
+)
+def test_is_blacklisted_value_for_io_streams(stream):
+    # The stream types report "_io" as their module, while MODULE_BLACKLIST lists "io".
+    assert type(stream).__module__ == "_io"
+    assert ato._is_blacklisted_value(stream) is True
+
+
 # --- RemoteAssertionTraceObserver: static class-field assertions --------------
 
 
diff --git a/tests/fixtures/examples/std_streams.py b/tests/fixtures/examples/std_streams.py
new file mode 100644
index 000000000..8b1dafc26
--- /dev/null
+++ b/tests/fixtures/examples/std_streams.py
@@ -0,0 +1,29 @@
+#  This file is part of Pynguin.
+#
+#  SPDX-FileCopyrightText: 2019–2026 Pynguin Contributors
+#
+#  SPDX-License-Identifier: MIT
+#
+"""Fixture whose objects keep references to the standard input streams."""
+
+import shlex
+import sys
+
+
+class StreamHolder:
+    """Keeps the text and binary standard input streams."""
+
+    def __init__(self) -> None:
+        """Store the currently bound stdin and its binary buffer."""
+        self.text = sys.stdin
+        self.binary = sys.stdin.buffer
+        self.name = "stdin"
+
+
+def create_lexer() -> shlex.shlex:
+    """Create a lexer that reads from the standard input.
+
+    Returns:
+        A lexer whose ``instream`` is ``sys.stdin``.
+    """
+    return shlex.shlex()