The black lines you are seeing here are known as edgelines. Essentially what is happening here is that the inside of the sleeves are coming outward. The easiest way to make it where those lines can't be seen is to go into your MMD tabs above. Go to "view" and in the drop down menu select "Thickness of Edgeline", once you select that a small box with a tiny tab in the center will appear in the middle of your MMD screen. If you drag that tab all the way to "0", all the edgelines on your model will disappear. If the box has the word "register" in it, be sure to click that after you move the edgeline thickness to "0".
Keep in mind that if you reduce a model's edgelines to "0" that any material that, for example, is like Animasa Miku's sleeves where you can see right through them, be sure to follow this tutorial:
fav.me/d4g0m7bThis same tutorial will also help with the sleeves too in case you want to keep your model's edgelines.