White arm: http://i.imgur.com/KTNky0O.png Obvious connection to sports, sunglasses(like saying "cool" in this context)
Black arm: http://i.imgur.com/uXtSRfc.png Policeman searching something, a location marker(search location?)
This is definitely a concern and something we've though about but not yet fully solved. The neural net is trained on real world data which unfortunately includes various types of questionable, racist, sexist, etc content. We already blacklisted emoji combination that are too often triggered in racist ways. However such a system is very difficult to audit completely.
Your example comparing different skin tone modifiers is a good one that we hadn't thought of. I've made a note of it so we can try and improve.
Although in this particular case it's actually just a bug: Dango gets confused by any skin tone modifier character, since they're not supported on Android (our target platform). Try putting in a "white" arm and you'll see the same results. They're actually just our "Dango is confused by this input" results.
We should fix the bug, of course!