Fix MarkdownRenderer blockquote child spacing

src/mistune/renderers/markdown.py

@@ -152,7 +152,7 @@ def block_quote(self, token: Dict[str, Any], state: BlockState) -> str:
         # strip the children's trailing blank lines first so the quote marker is
         # not added to a dangling empty line; stripping it back off afterwards
         # would also eat a ">" that ends the content (an autolink or HTML tag).
-        text = self.render_children(token, state).rstrip("\n")
+        text = _render_block_quote_children(self, token, state).rstrip("\n")
         text = indent(text, "> ", lambda _: True)
         return text + "\n\n"
 
@@ -216,6 +216,19 @@ def _escape_title(title: str) -> str:
     return title.replace("\\", "\\\\").replace('"', '\\"')
 
 
+def _render_block_quote_children(renderer: MarkdownRenderer, token: Dict[str, Any], state: BlockState) -> str:
+    children = token["children"]
+    parts: list[str] = []
+    previous_type = ""
+    for child in children:
+        child_type = child["type"]
+        if parts and previous_type != "blank_line" and child_type != "blank_line":
+            parts[-1] = parts[-1].rstrip("\n") + "\n"
+        parts.append(renderer.render_token(child, state))
+        previous_type = child_type
+    return "".join(parts)
+
+
 def _escape_block_prefix(text: str) -> str:
     """Backslash-escape a leading block marker on each line so that literal
     text is not re-parsed as a list, heading or block quote."""

tests/test_renderers.py

@@ -112,6 +112,13 @@ def test_block_quote_content_ending_in_gt(self):
         ):
             self.assert_round_trip(text)
 
+    def test_block_quote_adjacent_child_blocks(self):
+        reformat = create_markdown(renderer=MarkdownRenderer())
+        self.assertEqual(
+            reformat("> #### Lorem\n> Ipsum"),
+            "> #### Lorem\n> Ipsum\n",
+        )
+
     def test_multiline_setext_heading(self):
         # a setext heading may span several lines; re-emitting it as an ATX
         # heading ("# ...") drops every line after the first out of the