这叫歇后语！ “黑话”指的坏人与坏人（比如小偷，强盗之类）之间使用的，外人听不懂的话，这样的话就叫黑话。但是黑又可以理解成“黑色”、“没有灯光”看到不到东西也叫“黑”，于是“黑话”就从字面上理解成“没有灯光－－〉黑”，谈心要讲话，这样两个结合起来，就成了“黑+话”，本身这个黑话并不是“坏人用的黑话”，但是一说出来大家知道说话真正指的就是坏人讲的黑话，而不是“黑看不见的话”或“黑颜色的话”等等。同音不同义。 这种歇后语并不好用，用到句子里感觉太长，所以单独成句的时候要多些。 比如： 狗坐轿子——不知抬举。 抬举，是“称赞，提拔”的意思。 例子： 你真是“狗坐轿子——不知抬举”。 如果去掉“狗坐轿子”是一样的，“你真是不知抬举”是说话人真正要表达的。但是加上“狗坐轿子”就等于是在骂对方象狗一样。（中国人一般都认为把人比喻成狗是骂人的话）。 你提的这个歇后语： 几个坏蛋很狡猾，总是“谈心不点灯——讲的都是黑话”，外人根本听不懂他们在说什么。 看了大家的回复很受启发，没错，黑话也可以理解成见不得人的话，不想让人知道的话。但无论怎么说，黑话不是形容好话的词。

毛泽东同志曾说，说话不用点灯，留着这半只蜡晚上还得给报社写文章类！其明面上的意思是勤俭节约，暗含的意思是什么都可以说（可以瞎话）。 这是我的理解，也许还有更好的解释。 Comrade MAO zedong once said, don't light a lamp and keep this half candle night still have to write articles to the newspaper in class! The mean of the tooling is thrifty, implied meaning is what all can say. This is my understanding, there may be a better explanation of other people . ( sorry, I just speak a little English, Bad translation）

In German there is a saying: "Im Dunkeln ist gut munkeln." which basically means "Darkness(不点灯） is a good time for thieves and lovers." 'munkeln' could be any illicit or unseeming activity. However, I think '说黑语‘ expands on that and adds 'and saying what you will anonymously'。

Hey Joseph , only meaningful part is "说-黑话", the part before dash mark can be altered to anything that carry the same idea , one speaks with mouse full of ink is a good example, but here I guess "谈心" actually can strengthen the idea of "说-黑话" by contrast, because all we know about heart-to-heart talk is associated with qualities like straight, sincerity and honesty. as for 谈心不点灯 is easy to grasp the idea ,visually . "黑话", A--saying north but meaning south B--it's like those words we can get when we google gangster slang,con man slang, doctor slang ,child slang ,regional slang etc.. any obscure and figurative way of talking, code with hidden meaning ... C--it depends on the situation, itself as a word could be showing disapproval D--no better way learning it than Baidu searching '黑话' ,but it is a deep deep rabbit hole. 怎么用它？ it's good to know, but generally we won't use it on any occasion.

直白的说，就是偷偷聊一些不能被别人听到的话。内容可以是坏话，或是只能两个知道的悄悄话。