"dark art" meaning in English

{
  "forms": [
    {
      "form": "dark arts",
      "tags": [
        "plural"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {},
      "expansion": "dark art (plural dark arts)",
      "name": "en-noun"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
  "senses": [
    {
      "alt_of": [
        {
          "word": "black art"
        }
      ],
      "categories": [
        {
          "kind": "other",
          "name": "English entries with incorrect language header",
          "parents": [
            "Entries with incorrect language header",
            "Entry maintenance"
          ],
          "source": "w"
        }
      ],
      "examples": [
        {
          "ref": "2020 December 2, Andy Byford talks to Paul Clifton, “I enjoy really big challenges...”, in Rail, page 55",
          "text": "In New York, I learned the dark arts of politics, and spent a lot of time doing that.",
          "type": "quotation"
        }
      ],
      "glosses": [
        "Alternative form of black art"
      ],
      "id": "en-dark_art-en-noun-yWfje17v",
      "links": [
        [
          "black art",
          "black art#English"
        ]
      ],
      "tags": [
        "alt-of",
        "alternative"
      ]
    }
  ],
  "word": "dark art"
}

{
  "forms": [
    {
      "form": "dark arts",
      "tags": [
        "plural"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {},
      "expansion": "dark art (plural dark arts)",
      "name": "en-noun"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
  "senses": [
    {
      "alt_of": [
        {
          "word": "black art"
        }
      ],
      "categories": [
        "English countable nouns",
        "English entries with incorrect language header",
        "English lemmas",
        "English multiword terms",
        "English nouns",
        "English terms with quotations"
      ],
      "examples": [
        {
          "ref": "2020 December 2, Andy Byford talks to Paul Clifton, “I enjoy really big challenges...”, in Rail, page 55",
          "text": "In New York, I learned the dark arts of politics, and spent a lot of time doing that.",
          "type": "quotation"
        }
      ],
      "glosses": [
        "Alternative form of black art"
      ],
      "links": [
        [
          "black art",
          "black art#English"
        ]
      ],
      "tags": [
        "alt-of",
        "alternative"
      ]
    }
  ],
  "word": "dark art"
}