Index: wp-includes/pluggable.php
===================================================================
--- wp-includes/pluggable.php	(revision 22359)
+++ wp-includes/pluggable.php	(working copy)
@@ -1637,18 +1637,22 @@
 			$host = 'http://0.gravatar.com';
 	}
 
-	if ( 'mystery' == $default )
+	if ( 'mystery' == $default ) {
 		$default = "$host/avatar/ad516503a11cd5ca435acc9bb6523536?s={$size}"; // ad516503a11cd5ca435acc9bb6523536 == md5('unknown@gravatar.com')
-	elseif ( 'blank' == $default )
-		$default = includes_url('images/blank.gif');
-	elseif ( !empty($email) && 'gravatar_default' == $default )
+	} elseif ( 'blank' == $default ) {
+		if ( empty($email) ) {
+			$default = includes_url('images/blank.gif');
+		}
+		// else use '&d=blank' in the URL.
+	} elseif ( !empty($email) && 'gravatar_default' == $default ) {
 		$default = '';
-	elseif ( 'gravatar_default' == $default )
+	} elseif ( 'gravatar_default' == $default ) {
 		$default = "$host/avatar/?s={$size}";
-	elseif ( empty($email) )
+	} elseif ( empty($email) ) {
 		$default = "$host/avatar/?d=$default&s={$size}";
-	elseif ( strpos($default, 'http://') === 0 )
+	} elseif ( strpos($default, 'http://') === 0 ) {
 		$default = add_query_arg( 's', $size, $default );
+	}
 
 	if ( !empty($email) ) {
 		$out = "$host/avatar/";