The MacMillan Black Modern is not an official clan tartan as already stated. Tha confusion about the MacMillan tartans is understandable since not all manufacturers are as interrested in being official as they are interrested in making a buck.
The Ancient, The Hunting, and The Dress are the only official MacMillan tartans and certain variations of the three are accepted. If you or anyone reading this is still confused about what is an acceptable MacMillan tartan, you should know that The Clan MacMillan has an official website that can answer these questions and many others. The website is www.clanmcamillan.org.