"blind envelope" meaning in English

{
  "forms": [
    {
      "form": "blind envelopes",
      "tags": [
        "plural"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {},
      "expansion": "blind envelope (plural blind envelopes)",
      "name": "en-noun"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
  "senses": [
    {
      "categories": [
        {
          "kind": "other",
          "name": "English entries with incorrect language header",
          "parents": [
            "Entries with incorrect language header",
            "Entry maintenance"
          ],
          "source": "w"
        },
        {
          "kind": "topical",
          "langcode": "en",
          "name": "Marketing",
          "orig": "en:Marketing",
          "parents": [
            "Business",
            "Economics",
            "Society",
            "Social sciences",
            "All topics",
            "Sciences",
            "Fundamental"
          ],
          "source": "w"
        }
      ],
      "glosses": [
        "A plain envelope with no window showing the contents, used to make junk mail resemble legitimate correspondence."
      ],
      "id": "en-blind_envelope-en-noun-bm~NY94h",
      "links": [
        [
          "marketing",
          "marketing#Noun"
        ],
        [
          "window",
          "window"
        ],
        [
          "junk mail",
          "junk mail"
        ],
        [
          "legitimate",
          "legitimate"
        ],
        [
          "correspondence",
          "correspondence"
        ]
      ],
      "raw_glosses": [
        "(marketing) A plain envelope with no window showing the contents, used to make junk mail resemble legitimate correspondence."
      ],
      "topics": [
        "business",
        "marketing"
      ]
    }
  ],
  "word": "blind envelope"
}

{
  "forms": [
    {
      "form": "blind envelopes",
      "tags": [
        "plural"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {},
      "expansion": "blind envelope (plural blind envelopes)",
      "name": "en-noun"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
  "senses": [
    {
      "categories": [
        "English countable nouns",
        "English entries with incorrect language header",
        "English lemmas",
        "English multiword terms",
        "English nouns",
        "en:Marketing"
      ],
      "glosses": [
        "A plain envelope with no window showing the contents, used to make junk mail resemble legitimate correspondence."
      ],
      "links": [
        [
          "marketing",
          "marketing#Noun"
        ],
        [
          "window",
          "window"
        ],
        [
          "junk mail",
          "junk mail"
        ],
        [
          "legitimate",
          "legitimate"
        ],
        [
          "correspondence",
          "correspondence"
        ]
      ],
      "raw_glosses": [
        "(marketing) A plain envelope with no window showing the contents, used to make junk mail resemble legitimate correspondence."
      ],
      "topics": [
        "business",
        "marketing"
      ]
    }
  ],
  "word": "blind envelope"
}