Blackboard bold began as chalk improvisation and a typewriter workaround
Printed maths marks special number sets in bold, but chalk and typewriters cannot really do bold. So lecturers began doubling one stroke of a letter, and typists struck letters twice with a deliberate offset. The result, the hollow-looking R for real numbers or Z for integers, escaped the classroom and now appears in textbooks worldwide.
Before this, manuscripts flagged bold symbols with wavy underlines, circles or even wavy lines drawn above. Typewriters generally had no bold keys; overtyping a character made it darker but often not clearly distinct. The doubled-stroke style may have started in France in the late 1950s and caught on in Princeton's mathematics department in the early 1960s, with authors overtyping letters with a capital I to create new-looking symbols.
Printers resisted. It appeared in books from the mid-1960s, including Gunning and Rossi's 1965 text on complex variables, but most publishers stuck with ordinary bold. In 1979 Wiley urged authors to avoid what it called double-backed shadow letters, and by 1982 it refused them outright because the type was hard and costly to get. Donald Knuth disliked the style too and left it out of the Computer Modern fonts for TeX, released in 1978. When his 1984 TeXbook needed an example for its index, he faked one from an I and an R squeezed together, and in 1988 Robert Messer turned that trick into a full set of poor man's blackboard bold macros.
The American Mathematical Society produced a chalk-style typeface in 1985 and an inline version in 1990 to sit alongside Times New Roman. Unicode added the most common letters in its first version in 1991, inherited from a Xerox standard, and later extended the set to every Latin letter, plus digits and even Arabic letters.
Style guides slowly relented. The Chicago Manual of Style told writers in 1993 to keep blackboard bold in the classroom, then in 2003 accepted it for familiar number systems. Some holdouts remain: Jean-Pierre Serre writes it on the board but prints plain bold in his books.
Source: Blackboard bold
