I personally don't believe this is racists, but I am merely one person so I understand that others may feel differently.
I'd like to better understand why people believe it is racist. 'Blackface' has been traditionally deemed racist because of it's history and it's intent. It was typically done in a very dramatic fashion (ie. literally black skin, insanely large lips, etc) as a direct representation of 'Joe African American'. It was clearly racists in every way shape and form, as the entire 'character' was race. Blackface wasn't an attempt to portray an individual acurately, rather an excuse to paint an entire race of people with the same brush like some sort of caricature.
This appears to be the furthest thing from Blackface. This woman clearly loves cosplay, and the Walking Dead is a very popular series with cosplayers. We have evidence of her cosplaying in the past as another member of the cast, and we have an admission from her that Michonne is her favorite character. She and Michonne clearly have different skin colours, so she used makeup to try and make the cosplay more accurate.
This girl is not dressing as a caricature. She is not attempting to represent a race of people. For all intense and purposes, her cosplay was not about race at all but about accurately representing a character that she admires from a TV program.
I try to be very sensitive to all issues involving race, gender, sexuality, etc. I understand indirect racism, and how it can often times be more damaging than overt racism. I am aware of systemic prejudices that create large barriers for entire groups of people, and I know that even tangential comments can come from (even a slightly) racist root. However, I think this is even too tangential for my understanding. So once again, please share with me what is offensive about this girl's cosplay? What is the racist root that we are exposing? When something is this far off tangent from direct racism, does intent not come into play?
I am genuinely asking because I really do want to understand.