Mesh is black when imported to S4 studio

Oh. My. God. I forgot about that XD. But there’s also a problem, I used a swimming suit to use for a base considering there’s no full body clothing nude base and I don’t know how to remove it.

Edit: It still remains black... It has to be an issue in my blender process

Texture is there, but uh... 

Looking at that screenshot I would say that your UV mapping does not line up with the texture. You need to do this in blender.
The texture also seems very tiny compared to a regular Sims 4 clothing texture.

You have to resize the uv in uv_0 to get it to fit. the best way to do that is to first shrink it down by using the following commands in blender on the uv side.
Press s and then press y don't move it to shrink it. While you still have it in scale press the following keys. 0.5 Then move it over the top of your texture. Shrink it a little more and in order to get it to fit you have to scale it with s plus x axis and s plus y axis until you can get it to fit.

Quick question, before you merged the meshes, did you create uv_0 and uv_1 maps on the xps mesh? You also have to delete the material on the xps mesh before merging it other wise you will see black and not the sim skin texture.

You have the wrong UV selected. In the lower right of the screen you have selected uv_1. That is for controlling the morphing of the body. That is why the body mesh looks mirrored. You need to have uv_0 selected when working with your textures.