What is the meaning of Black_letter?

A highly calligraphic Western European script style used from approximately 1150.

2. A Northern European style of type based on the script, with contrasting thick-and-thin, angular strokes forming upright letterforms, and usually set with a dark typographic colour on the page.

   A Northern European style of type based on the script, with contrasting thick-and-thin, angular strokes forming upright letterforms, and usually set with a dark typographic colour on the page.

   Text set in black-letter type.

   The basic standard elements for a particular field of law, which are generally known and free from doubt or dispute.

   Source: wiktionary.org