Check out RaptureXML. It's not official XPath behavior, but it's pretty close. Your query would be:
[rxml iterate:@"comments.comment" with: ^(RXMLElement *e) {
if ([[e child:@"author"].text isEqualToString:@"foo1"]) {
NSLog(@"Blog Text is %@.", e.text);
} else {
NSLog(@"Comment Text is %@.", e.text);
}
}];
UPDATE: RaptureXML now supports XPath!