Black coffee = a cup of coffee without added milk
Dark coffee = only refers to the colour (as far as I know)
Bitter coffee = only refers to the taste (sounds disgusting)
(You might want to bear in mind that I don't drink coffee so these terms might have other meanings to coffee-drinkers.)
I can speak far more knowledgeably about chocolate terminology! Dark chocolate usually refers to chocolate with more than 60% cocoa. Bitter chocolate is harder to define. I know some people who think that 70%-80% cocoa chocolate tastes bitter but that might be because they are used to eating sugar-laden milk chocolate. I enjoy up to 95% cacao chocolate and don't find it bitter at all.
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