Someone sent me this:
And when I translated it sounded offensive.
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The Google Translation of this sentence is direct and quite accurate:
All of these plastic bags come from neighboring African countries because they often use this black bag.
It breaks down as follows:
As far as I can tell (bearing in mind that I'm just a learner), it's not directly rude. The "black" only refers to color of the plastic bags, and I don't see any connection with race.
I guess it implies African people in the nearby country (or countries) tend to be litterbugs, and they allow their rubbish to blow into neighboring countries, which might be a bit insulting.