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.
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.
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).
You can't have a transparent skin.
isn't this the same as 48294?