"black mud" meaning in English

{
  "forms": [
    {
      "form": "black muds",
      "tags": [
        "plural"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {
        "1": "~"
      },
      "expansion": "black mud (countable and uncountable, plural black muds)",
      "name": "en-noun"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
  "senses": [
    {
      "glosses": [
        "Used other than figuratively or idiomatically: see black, mud."
      ],
      "id": "en-black_mud-en-noun-~prTGG9H",
      "links": [
        [
          "black",
          "black#English"
        ],
        [
          "mud",
          "mud#English"
        ]
      ],
      "tags": [
        "countable",
        "uncountable"
      ]
    },
    {
      "categories": [
        {
          "_dis": "4 96",
          "kind": "other",
          "name": "English entries with incorrect language header",
          "parents": [
            "Entries with incorrect language header",
            "Entry maintenance"
          ],
          "source": "w+disamb"
        }
      ],
      "glosses": [
        "Reduced mud found below the surface of sediments in lakes and rivers, the blackness of which is caused by the occurrence of metal sulphides."
      ],
      "id": "en-black_mud-en-noun--NdK9HGC",
      "links": [
        [
          "Reduce",
          "reduce"
        ],
        [
          "sediment",
          "sediment"
        ],
        [
          "sulphide",
          "sulphide"
        ]
      ],
      "tags": [
        "countable",
        "uncountable"
      ]
    }
  ],
  "word": "black mud"
}

{
  "categories": [
    "English countable nouns",
    "English entries with incorrect language header",
    "English lemmas",
    "English multiword terms",
    "English nouns",
    "English uncountable nouns"
  ],
  "forms": [
    {
      "form": "black muds",
      "tags": [
        "plural"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {
        "1": "~"
      },
      "expansion": "black mud (countable and uncountable, plural black muds)",
      "name": "en-noun"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
  "senses": [
    {
      "glosses": [
        "Used other than figuratively or idiomatically: see black, mud."
      ],
      "links": [
        [
          "black",
          "black#English"
        ],
        [
          "mud",
          "mud#English"
        ]
      ],
      "tags": [
        "countable",
        "uncountable"
      ]
    },
    {
      "glosses": [
        "Reduced mud found below the surface of sediments in lakes and rivers, the blackness of which is caused by the occurrence of metal sulphides."
      ],
      "links": [
        [
          "Reduce",
          "reduce"
        ],
        [
          "sediment",
          "sediment"
        ],
        [
          "sulphide",
          "sulphide"
        ]
      ],
      "tags": [
        "countable",
        "uncountable"
      ]
    }
  ],
  "word": "black mud"
}