Serge Lang also used boldface instead of blackboard bold in his highly influential Algebra. The Chicago Manual of Style evolved over this issue. In 1993, for the 14th edition, it advised that "blackboard bold should be confined to the classroom" (13.14).
Blackboard bold is a typeface style that is often used for certain symbols in mathematical texts, in which certain lines of the symbol (usually vertical or near-vertical lines) are doubled.
In some texts, these symbols are simply shown in bold type. Blackboard bold in fact originated from the attempt to write bold letters on blackboards in a way that clearly differentiated them from non-bold letters (by using the edge rather than the point of a chalk). It then made its way back into print form as a separate style from ordinary bold, possibly starting with the original 1965 edition of Gunning and Rossi's textbook on complex analysis.