User blog comment:Lord Loss/Monster Hunter Theory... Blog/@comment-25192100-20140908182425/@comment-11537809-20140908185416

Just did a quick search on Black Diablos, the blue markings are there in 3U screenshots I saw (although I can't find any of the green markings you speak of in Diablos' 4th gen render but I did find some small shades of green on the 3rd gen render). So these markings may just be a part of the "new" Diablos models and are caused by light reflecting on their scales.