Before the black figures were implemented, a player whose textures were not loaded yet on your client was plain invisible for you. He could attack and kill you in that timeframe without you knowing wtf is happening. Not very fun for the recipient. Thus, black outlines.
Yep, this is working as intended! And yes, it is to reduce loading times. And I am so glad they did this! As I have understood it, it's a mechanism that prioritizes all other textures BEFORE loading the other players, which means the world gets loaded quicker = reduced loading times/screens.