diff --git a/test-requirements.in b/test-requirements.in
index f9f6b9a..011bf5a 100644
--- a/test-requirements.in
+++ b/test-requirements.in
@@ -5,7 +5,7 @@ pytest-trio
 pytest-faulthandler
 
 # Tools
-black == 19.10b0
+black == 19.10b0; implementation_name == "cpython"
 mypy >= 0.750; implementation_name == "cpython"
 flake8
 
@@ -15,7 +15,7 @@ async_generator >= 1.9
 trio >= 0.12.0
 trio-typing >= 0.3.0
 
-# typed-ast is required by mypy and doesn't build on PyPy;
+# typed-ast is required by black + mypy and doesn't build on PyPy;
 # it will be unconstrained in requirements.txt if we don't
 # constrain it here
 typed-ast; implementation_name == "cpython"
diff --git a/test-requirements.txt b/test-requirements.txt
index 424d693..804a43e 100644
--- a/test-requirements.txt
+++ b/test-requirements.txt
@@ -7,7 +7,7 @@
 appdirs==1.4.3            # via black
 async-generator==1.10
 attrs==19.3.0
-black==19.10b0
+black==19.10b0 ; implementation_name == "cpython"
 click==7.0                # via black
 contextvars==2.4          # via sniffio, trio
 coverage==4.5.4           # via pytest-cov