My understanding of MedievalPOC is that it's pretty problematic for a few reasons:
- Presentism. In this case, the blogger in question is applying modern day constructs on what it means to be "black", "white", "person of color", etc. to the past. What it means to be "white" today does not have the same meaning as what it meant to be "white" in the past, presuming that people during this time period (1100s-1700s according to the blog) even had a concept of what is "white". So we're already starting off with a problematic assumption that modern day concepts can apply to the past.
- Misrepresenting historical evidence and manipulating them to push an agenda. There is at least one post I'm thinking of where the blogger posted about Prince Edward of Woodstock, nowadays popularly known as "the Black Prince". They claimed that said person was black and that they were called "the Black Prince" during their lifetime and that his wife was actually his mother. For the record, here's the Wikipedia page.
- Outright lying. They once claimed that Beethoven was black, despite all of the evidence against it (like, you know, actual portraits of the dude).
More information can be found in the following threads on /r/badhistory: [1] [2] [3]
Hope this helps.