Without knowing anything about the particular person, in what ways a black thumbs-up is different than a non-skinned thumbs-up?

Adding a skin-tone is saying there ARE differences, but any difference you can name are actually prejudices about blacks. What was meant to be a simple "ok", "agree", etc. now is charged with an "I don't think you/others and me are the same kind of humans".

That's why using skin-tones in emojis is actually racist.