I literally just created an account just to reply to you.
Blackbeard can create, manipulate, and become darkness, hence he is a logia.
Yami Yami no mi is considered a "special" fruit logia, so it's an exception, so can't be used as a counter-example.
the same applies for katakuri, his fruit is considered a "special" paramecia type, so you can't use those in your arguments.
and as to why Robin's fruit isn't a logia, it's simple, she can't 'turn' herself into human body parts. she can only create body parts.
to define logia types, I prefer using this rule : if you can't damage the user without using haki, it's a logia, and if you can damage the user without using haki, it's not a logia.
Katakuri only uses his future sight to "dodge" attacks, but technically you can damage him without haki.
You could damage luffy without haki too even if he's rubber.