Transparent parts of the skin look black in game

You can't have a transparent skin.

I thought you used to be able to do it. When did they change it?

(my legs arent transparent, only my arms)

This has always been the case because you could be entirely invisible and people can't see you at all.You wouldn't want invisible people running around killing you and destroying your buildings if PvP was enabled. The only parts that can be invisible are your second layers in the new skin design (and the hat in previous versions).

But again, its only my arms that are invisible, everything else is visible. I dont see how that would be a problem.

What if I make the arms only 50% transparent, would that work? Or would it still look 100% black?

Nothing can be transparent except from the new extra layers and the hat. If your arms were 50% transparent then they would be 50% black. There is no way of getting invisible arms on your skin.

Yeah thats what I meant, if I make my arms 50% transparent / 50% black, will they look translucent or will they just be opaque?

They will look opaque. As I said there is no way of getting invisible arms. You could ask Mojang to change this, however they probably will ignore you.

This site is for bug reports only. For feature suggestions or changes please see: Minecraft Suggestions on Reddit.

And yes, Mojang does look at the suggestions site from time to time, there are a fair few features that originated in those posts.

I didnt post this as a suggestion, I posted this because I didnt know you couldnt have transparency in skins, so I thought it was a bug.

So, no need to tell me this site is for bug reports only. I already know that.