Yes that comment was offensive on youtube, but remember race and hair is a touchy subject. So for you to bring the negative comment here, and then say "being born black does not mean that you have all the knowledge on how to look after mixed kids hair, it is very different to "black" hair!" Your just adding fuel to the negative comment.
I know you wanted to vent, but in life you are going to find ignorant comments/people EVERYWHERE lol. That's why it is best to address it to them.