diff --git a/CHANGES.md b/CHANGES.md
index 440aaeaa35a..274c5640ec0 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -23,6 +23,7 @@ and the first release covered by our new stability policy.
 - Use parentheses for attribute access on decimal float and int literals (#2799)
 - Don't add whitespace for attribute access on hexadecimal, binary, octal, and complex
   literals (#2799)
+- Treat blank lines in stubs the same inside top-level `if` statements (#2820)
 
 ### Parser
 
diff --git a/src/black/lines.py b/src/black/lines.py
index 7d50f02aebc..1c4e38a96c1 100644
--- a/src/black/lines.py
+++ b/src/black/lines.py
@@ -530,11 +530,11 @@ def _maybe_empty_lines_for_class_or_def(
             return 0, 0
 
         if self.is_pyi:
-            if self.previous_line.depth > current_line.depth:
-                newlines = 0 if current_line.depth else 1
-            elif current_line.is_class or self.previous_line.is_class:
-                if current_line.depth:
+            if current_line.is_class or self.previous_line.is_class:
+                if self.previous_line.depth < current_line.depth:
                     newlines = 0
+                elif self.previous_line.depth > current_line.depth:
+                    newlines = 1
                 elif current_line.is_stub_class and self.previous_line.is_stub_class:
                     # No blank line between classes with an empty body
                     newlines = 0
@@ -551,6 +551,8 @@ def _maybe_empty_lines_for_class_or_def(
                     # Blank line between a block of functions (maybe with preceding
                     # decorators) and a block of non-functions
                     newlines = 1
+            elif self.previous_line.depth > current_line.depth:
+                newlines = 1
             else:
                 newlines = 0
         else:
diff --git a/tests/data/stub.pyi b/tests/data/stub.pyi
index 9a246211284..af2cd2c2c02 100644
--- a/tests/data/stub.pyi
+++ b/tests/data/stub.pyi
@@ -32,6 +32,48 @@ def g():
 
 def h(): ...
 
+if sys.version_info >= (3, 8):
+    class E:
+        def f(self): ...
+    class F:
+
+        def f(self): ...
+    class G: ...
+    class H: ...
+else:
+    class I: ...
+    class J: ...
+    def f(): ...
+
+    class K:
+        def f(self): ...
+    def f(): ...
+
+class Nested:
+    class dirty: ...
+    class little: ...
+    class secret:
+        def who_has_to_know(self): ...
+    def verse(self): ...
+
+class Conditional:
+    def f(self): ...
+    if sys.version_info >= (3, 8):
+        def g(self): ...
+    else:
+        def g(self): ...
+    def h(self): ...
+    def i(self): ...
+    if sys.version_info >= (3, 8):
+        def j(self): ...
+    def k(self): ...
+    if sys.version_info >= (3, 8):
+        class A: ...
+        class B: ...
+        class C:
+            def l(self): ...
+            def m(self): ...
+
 
 # output
 X: int
@@ -56,3 +98,54 @@ class A:
 
 def g(): ...
 def h(): ...
+
+if sys.version_info >= (3, 8):
+    class E:
+        def f(self): ...
+
+    class F:
+        def f(self): ...
+
+    class G: ...
+    class H: ...
+
+else:
+    class I: ...
+    class J: ...
+
+    def f(): ...
+
+    class K:
+        def f(self): ...
+
+    def f(): ...
+
+class Nested:
+    class dirty: ...
+    class little: ...
+
+    class secret:
+        def who_has_to_know(self): ...
+
+    def verse(self): ...
+
+class Conditional:
+    def f(self): ...
+    if sys.version_info >= (3, 8):
+        def g(self): ...
+    else:
+        def g(self): ...
+
+    def h(self): ...
+    def i(self): ...
+    if sys.version_info >= (3, 8):
+        def j(self): ...
+
+    def k(self): ...
+    if sys.version_info >= (3, 8):
+        class A: ...
+        class B: ...
+
+        class C:
+            def l(self): ...
+            def m(self): ...